He is the founder of the Hypatia Academy Cyprus, an online school to teach secondary school children programming. Otherwise the first field is the partition key. There are a number of columns in a row but the number of columns can vary in different rows. Clustering columns determines the order of data in partitions. Each Cassandra table has a partition key which can be standalone or composite. The use of a partition key is to determine the partition in the cluster that stores that row. cqlsh:students_details> select * from student; We can see from the above output that the stuid has become the row key, and it identifies individual rows. With either method, we should get the full details of matching user. Table Partitioning : In table partitioning, data can be distributed on the basis of the partition key. (C1,C2,C3,…): Column C1 is a partition key and columns C2, C3, and so on make the cluster key. Figure 2. This hash value is used to determine the node/partition which contains that row. But let’s suppose they do not need to be for these examples. In order to make composite partition keys, we have to specify keys in parenthesis such as: ( ( C1,C2) , C3, C4). The other concept that needs to be taken into … In Cassandra, primary keys can be simple or compound, with one or more partition keys, and optionally one or more clustering keys. But in a column oriented database one row can have columns (a,b,c) and another (a,b) or just (a). Before we dive into the basic rules of data modelling in Cassandra, let us quickly look at what these terms mean, Partition. A partition key is used to partition data among the nodes. I will explain to you the key points that need to be kept in mind when designing a schema in Cassandra. The Cassandra API for Azure Cosmos DB allows up to 20 GB per partition. He writes tutorials on analytics and big data and specializes in documenting SDKs and APIs. Two-part primary key I’ll use this shorthand pattern to represent the primary key: ((AB)CDE). Cassandra Primary Key = ((Partitioning Key), Clustering Key): A simple explanation Cassandra primary key (a unique identifier for a row) is made up of two parts – 1) one or more partitioning columns and 2) zero or more clustering columns. 2 thoughts on “ Cassandra : Primary key vs Partition key vs Clustering key vs composite key ” Spille says: May 25, 2016 at 10:32 pm Nice short explanation. Run cassandra-cli in a separate terminal windo. SSTables A generic diagram that (I hope) summarize ! Join the DZone community and get the full member experience. (This article is part of our Cassandra Guide. This is just a table with more than one column used in the calculation of the partition key. All the data associated to that partition key is stored as columns in the datastore. In this case isbn and author are the partition key and publisher is a clustering key. Table Partitioning in Cassandra Last Updated: 31-08-2020. The purpose of the partition key is to identify the node that has stored that particular row which is being asked for. Add some data into the table: cqlsh:students_details> select * from marks; Now, let's see how the partition concept has been applied: cqlsh:students_details> select token(stuid) from marks; We can see all the three rows have the same partition token, hence Cassandra stores only one row for each partition key. Apache Cassandra allows you to disable durable commits. These postings are my own and do not necessarily represent BMC's position, strategies, or opinion. Type the following command in the cqlsh window: Now we will create another table called marks, which records marks of each student every day (say every day, new exams and marks are recorded). We can see how Cassandra has stored this data under the hood by using the cassandra-cli tool. Remember than in a regular rdbms database, like Oracle, each row stores all values, including empty ones. Partition key and Clustering key are the terms that anyone dealing with Cassandra should be aware of. Hashing is a technique used to map data with which given a key, a hash function generates a hash value (or simply a hash) that is stored in a hash table. Important: The CLI utility is deprecated and will be removed in Cassandra 3.0. The primary key concept in Cassandra is different from relational databases. With primary keys, you determine which node stores the data and how it partitions it. DynamoDB’s data model: Here’s a simple DynamoDB table. The purpose of a partition key is to identify the partition or node in the cluster that stores that row. In this case we have three tables, but we have avoided the data duplication by using last two tabl… The partition key is responsible for distributing data among nodes. In DynamoDB, it’s possible to define a schema for each item, rather than for the whole table. Architecture. Now switch to the students_details keyspace: Check the number of tables present in the keyspace: We will create a table, student , that contains general information about any student. The inner parentheses enclose the partition key column(s), and clustering columns follow. A Cassandra Primary Key consists of two parts: the partition key and the clustering column list. cqlsh:students_details> select token(stuid) from student; Also, you can see that there are two tokens. For example, rows whose partition key values range from 1000 to 1234 may reside in node A, and rows with partition key values range from 1235 to 2000 may reside in node B, as shown in figure 1. Cassandra’s hard limit is 2 billion cells per partition, but you’ll likely run into performance issues before reaching that limit. In this case, a partition key performs the same functio… See the original article here. Another way to model this data could be what’s shown above. Each row is referenced by a primary key, also called the row key. The desi… The following are different variations of primary keys. Note that the primary key is PRIMARY KEY (isbn, author, publisher). When an index query is performed, Casssandra will retrieve the primary keys of the rows containing the value from the index. It is responsible for data distribution across the nodes. For performance reasons choose partition keys whose number of possible values is bounded. Opinions expressed by DZone contributors are their own. This arrangement makes it efficient to retrieve data using the clustering key. Any fields listed after the partition key are called clustering columns. 2. Using the EXPAND Command in cqlsh , we can view the details info for the queries . Stress Testing and Performance Tuning Apache Cassandra, Configuring Apache Cassandra Data Consistency, Using Tokens to Distribute Cassandra Data, Prev: Using Tokens to Distribute Cassandra Data. If expanded output is disabled. There are two types of primary keys: It will then retrieve the rows from the table and perform any filtering needed on it. In this post, we are going to discuss the different keys available in Cassandra. Here we explain the differences between partition key, composite key and clustering key in Cassandra. All we have changed with the compound key is the calculation of the partition key and thus where the data is stored. (C1, (C2, C3,…)): It is same as 3, i.e., column C1 is a partition key and columns C2,C3,… make the cluster key. The partition size is a crucial attribute for Cassandra performance and maintenance. Now to show the partition key value we use the SQL token function and give it both the isbn and author values: Add the same data as above with the insert SQL statements. I hope these examples have helped you to clarify some of the concepts of data modeling in Cassandra. By definition the primary key must be unique. In the extreme case, you can have one partition per row, so you can easily have billions of these entries on a single machine. What virtual node it is stored on depends on the token range assigned to the virtual node. The purpose of the clustering key is to store row data in a sorted order. samarthmaiya says: February 20, 2017 at 10:23 am Nice blog. Remember that SQL select statements create subsets. The partition key has a special use in Apache Cassandra beyond showing the uniqueness of the record in the database. This can lead to data loss if the node goes down before memtables are flushed to SSTables on disk. The partition key is responsible for distributing data among nodes. We start with very basic stats and algebra and build upon that. In this article, we are going to cover how we can our data access on the basis of partitioning and how we can store our data uniquely in a cluster. Notice that adding this data also drops one book because one author wrote more than one book with the same ISBN. Like Like. Therefore, it is worth spending some time to understand it. Use EXPAND ON to enable it. In the solutions, columns in the logical and actual table primary key definitions are in the order presented. And if the primary key is composite, it consists of both a partition key and a sort key. Let’s look at books. As the name suggests, a compound primary key is comprised of one or more columns that are referenced in the primary key. Partition Key Cache. Picking the right data model is the hardest part of using Cassandra. The whole point of a column-oriented database like Cassandra is to put adjacent data records next to each other for fast retrieval. Normally it is a good approach to use secondary indexes together with the partition key, because - as you say - the secondary key lookup can be performed on a single machine. Partition Key:-Data in Cassandra is spread across the nodes. 5. C1: Primary key has only one partition key and no cluster key. Cassandra performs these read and write operations by looking at a partition key in a table, and using tokens (a long value out of range -2^63 to +2^63-1) for data distribution and indexing. 1, therefore, all the data is saved in that row as columns, i.e under one partition. In this case, all the columns, such as exam_name and marks, will be grouped by value in exam_date, i.e 2016-11-11 18:30:00+0000, by default in ascending order . Published at DZone with permission of Piyush Rana, DZone MVB. The important elements of the Cassandra partition key discussion are summarized below: 1. In above table, Car name is a partitioning key. In order to calculate the size of partitions, use the following formula: \ [N_v = N_r (N_c - N_ {pk} - … Also, Cassandra’s primary key contains partition key and the clustering columns in which the partition key might contain different columns. Cassandra Introduction: What is Apache Cassandra? We can use columns in the primary key to filter data in the select statement. BloomFilter (for each SSTable) 4. These store data in ascending or descending order within the partition for the fast retrieval of similar values. Use the right-hand menu to navigate.). (We discussed keyspaces here.). Here we explain the differences between partition key, composite key and clustering key in Cassandra. So the column-oriented approach makes the prime data structure a type of subset. However, if the query expects a result set instead of a deterministic row like below. Let’s discuss one by one. Key cache 5. primary_key((partition_key), clustering_col ) 1. Please feel free to leave any comments. Limits the Size of Partitions. Learn more about BMC ›. It looks like Cassandra relies on the Partitioner and Replication Strategy to process queries. Partitions are formed based on the value of a partition key that is associated with each record in a table. Let's take an example and create a student table which has student_id as a primary key column. All the data that is inserted against same clustering key is grouped together. This hash value is used to calculate the partition in the row. Notice that all of the values in the primary key must be unique, so it dropped one record because author Fred wrote and published more than one book with published Penguin Group. I am still confused that the Partitioner needs to know the Partition Key. One part of that key then called Partition Key and rest a Cluster Key. You can skip writing to the commit log and go directly to the memtables. The ISBN is a serial number of a book used by publishers. Using partition key along with secondary index. Create a books keyspace, table, and put some data into it. Now view the details inserted above (the studid will be present in a red color in cqlsh, representing the primary key/row key). Primary key is comprised of a partition key plus clustering columns, if any, and uniquely identifies a row in both its partition and table : Row (Partition) Row is the smallest unit that stores related data in Cassandra . The key thing here is to be thoughtful when designing the primary key of a materialised view (especially when the key contains more fields than the key of the base table). The additional columns determine per-partition clustering. Reply. The first field listed is the partition key, since its hashed value is used to determine the node to store the data. As the primary key has two components, the first component is considered a partition key, and the second component becomes the cluster key. Each table has a primary key, which can be either simple or composite. Its rows are items, and cells are attributes. When data is read or written from the cluster, a function called Partitioner is used to compute the hash value of the partition key. So if we are only interested in the value a then why not store that in the same data center, rack, or drive for fast retrieval? Choose a partition key that has a high cardinality to avoid hot spots—a situation where one or a few nodes are under heavy load while others are idle. A partition key is generated from the first field of a primary key. Walker Rowe is an American freelancer tech writer and programmer living in Cyprus. Checks if the in-memory memtable cache still contain the data (if it is not yet flushed to SSTable) 2. The Bloom filter is tunable if you want to trade memory for performance. In Cassandra, we can only access data from the partitioning key. For ease of use and performance, switch from Thrift and CLI to CQL and cqlsh.). Each key cache entry is identified by a combination of the keyspace, table name, SSTable, and the Partition key. This approach makes logical sense since we are usually only interested in a part of the data at any one time. To make these concepts clear, we will consider the example of a school system. Distributes Data Evenly Around the Cassandra Cluster. Over a million developers have joined DZone. When data is inserted into the cluster, the first step is to apply a hash function to the partition key. So if you're using a Cassandra verison above 3.0, then use the below commands. In brief, each table requires a unique primary key. Apache Cassandra recommends a 100-MB limit on the size of a partition key. It is important to note that when the compound key is C1, C2, C3, then the first key, C1, becomes the partition key, and the rest of the keys become part of the cluster key. EXPAND with no arguments shows the current value of the expanded setting. The partition key determines data locality through indexing in Cassandra. In brief, each table requires a unique primary key. Observe again that the data is sorted on the cluster columns author and publisher. 3) Partition Key The purpose of a partition key is to identify the partition or node in the cluster that stores that row. The data that we have stored through three different insert statements have the same stuid value, i.e. 3. It is also important to note that in Cassandra, both column names and values have binary types. If the query has the Paritition Key, the internal query process looks straightforward. Cassandra is a distributed database in which data is partitioned and stored across different nodes in a cluster. In Cassandra, on one hand, a table is a set of rows containing values and, on the other hand, a table is also a set of partitions containing rows. If those fields are wrapped in parentheses then the partition key is composite. One component of the compound primary key is called partition key, whereas the other component is called the clustering key. We saw that student_id was used as a row key to refer to person data. For example, one row in a table can have three columns whereas another row in the same table can have 10 columns. And the token is different for the 333 primary key value. Type the following command on cqlsh: This statement creates the marks table with a primary key (stuid , exam_date ). For example why retrieve employee tax IDs, salary, manager’s name, when we just want their name and phone number? Now add another record but give it a different primary key value, which could result it in being stored in a different partition. Partition keys belong to a node. In Cassandra, a table can have a number of rows. Note that we are duplicating information (age) in both tables. The first field listed is the partition key, since its hashed value is used to determine the node to store the data. This e-book teaches machine learning in the simplest way possible. Partition Summary 6. They are supposed to be unique. For each indexed value, Cassandra stores the full primary key (partition key columns + clustering columns) of each row containing the value. ((C1, C2,…), (C3,C4,…)): columns C1, C2 make partition key and columns C3,C4,… make the cluster key. The partition key determines which node stores the data. cassandra, nosql, bigdata, cassandra-2.0 Normally it is a good approach to use secondary indexes together with the partition key, because - as you say - the secondary key lookup can be performed on a single machine. The other purpose, and one that very critical in distributed systems, is determining data locality. This book is for managers, programmers, directors – and anyone else who wants to learn machine learning. Clustering is a storage engine process that sorts data within the partition. Pre-requisite — Data Distribution. Cassandra is classified as a column based database which means that its basic structure to store data is based on a set of columns which is comprised by … If you remember, we discussed before that the second component of a primary key is called the clustering key. Cassandra is organized into a cluster of nodes, with each node having an equal part of the partition key hashes. Recall that the partitioner has function configured in cassandra.yaml calculated the hash value and then distributes the data based upon partitioner. The ideal size of a Cassandra partition is equal to or lower than 10MB with a maximum of 100MB. The Bloom filter grows to approximately 1-2 GB per billion partitions. Partition Key vs Composite Key vs Clustering Columns in Cassandra, ©Copyright 2005-2020 BMC Software, Inc. Create a keyspace with replication strategy ‘SimpleStrategy’ and replication_factor 1. If the primary key is simple, it contains only a partition key that defines what partition will physically store the data. cassandra,nosql,bigdata,cassandra-2.0. What is the reason for having clustering columns? 1 therefore all the data is saved in that row as columns. 2. All the fields together are the primary key. The default is org.apache.cassandra.dht.Murmur3Partitioner. Specifically, each row belongs to exactly one partition and each partition contains one or more rows. Please note that C1, C2, C3,… and so on represent columns in the table. This primer is meant to be enough to understand key designs in the solutions and a little more. Cassandra Data Modeling: Primary, Clustering, Partition, and Compound Keys, Developer If the Bloom filter does not rule out an SSTable, Cassandra checks the partition key cache. All the records in a partition have the same partition key value. Use of this site signifies your acceptance of BMC’s, How To Write Apache Spark Data to ElasticSearch Using Python, Outlier and Anomaly Detection with Machine Learning, How to Configure Filebeat for nginx and ElasticSearch, Introduction to TensorFlow and Logistic Regression, How To Import Amazon S3 Data to Snowflake. When data is read or written from the cluster, a function called Partitioned.It is used to calculate the hash value of the partition key. Cassandra partitions data over the storage nodes using a variant of consistent hashing for data distribution. Cassandra relies on the partition key to determine which node to store data on and where to locate data when it's needed. A partition key is the same as the primary key when the primary key consists of a single column. The update in the base table triggers a partition change in the materialised view which creates a tombstone to remove the row from the old partition. You can find Walker here and here. From core to cloud to edge, BMC delivers the software and services that enable nearly 10,000 global customers, including 84% of the Forbes Global 100, to thrive in their ongoing evolution to an Autonomous Digital Enterprise. This is different from SQL databases, where each row in a SQL table has a fixed number of columns, and column names can only be text. In first implementation we have created two tables. We denote that with parentheses like this: PRIMARY KEY ((isbn, author), publisher). Please let us know by emailing blogs@bmc.com. Before detailing the cache working, we have to dig in reading path : First, two drawing (from datastax website) to represent it : So what I understand : 1. If a row contains partition key whose hash value is 1233 then it will be stored in node A. 4. It would make sense that in a collection of books you would want to store them by author and then publisher. The data which we have stored through three different insert statements have the same cityid value i.e. All the data associated with that partition key is stored as columns in the datastore. Consider a scenario where we have a large number of users and we want to look up a user by username or by email. That means column names can have binary values, such as strings, timestamps, or an integer, etc. The partition key should be designed carefully to create boun… Order matters! That includes clustering columns, since they are part of the primary key. A function, called partition, is used to compute the hash value of the partition key at the time of row is being written. Marketing Blog. Reply. In this case, C1 and C2 are part of the partition keys, and C3 and C4 are part of the cluster key. See an error or have a suggestion? Partition size is measured by the number of cells (values) that are stored in the partition. Cassandra Primary Key Types Every table in Cassandra needs to have a primary key, which makes a row unique. Cassandra’s key cache is an optimization that is enabled by default and helps to improve the speed and efficiency of the read path by reducing the amount of disk activity per read. Leave a Reply Cancel reply. One of the Cassandra key characteristics is that it only allows for a primary key to have multiple columns and HBase only comes with 1 column row keys and puts the responsibility of the row key design on the developers. Partition key - The first part of the primary key. If enabled, row cache 3. They are all the same since we want them all stored on the same virtual node. Now select all records and notices that the data is sorted by author and then publisher within the partition key 111. Like Like. The reason is that Cassandra stores only one row for each partition key. Cassandra API uses partitioning to scale the individual tables in a keyspace to meet the performance needs of your application. (C1, C2): Column C1 is a partition key and column C2 is a cluster key. The primary key in Cassandra usually consists of two parts - Partition key and Clustering columns. Type the following insert statements to enter some data into this table. The sorting of data is based on columns, which are included in the clustering key. The role of the clustering key is to group related items together. Now select the partition key and the primary key. Maybe you should mention, that Primary Key is the combination of Partition Key and Clustering Key. One has partition key username and other one email. In this case isbn is the partition key and author and publisher are clustering keys. Build upon that here we explain the differences between partition key value function! Organized into a cluster see how Cassandra has stored this data also drops one book because author! Possible values is bounded is for managers, programmers, directors – anyone... Modeling in Cassandra in above table, Car name is a clustering key model is the same cityid value.. Columns determines the order of data is sorted by author and then distributes the data with! In cqlsh, we are usually only interested in a table can a! Point of a deterministic row like below be stored in a table an integer, etc purpose... You the key points that need to be for these examples showing the uniqueness of the Academy! They do not necessarily represent BMC 's position, strategies, or integer. Locate data when it 's needed s primary key use the below commands same table can binary! Possible to define a schema in Cassandra table name, when we just their... Can only access data from the index filtering needed on it access data from the first step is to the! The Partitioner needs to know the partition key - the first field listed is the isbn... Different primary key concept in Cassandra, both column names and values have binary values, such as strings timestamps... Or more rows and maintenance documenting SDKs and APIs first part of the partition key.... Of cassandra partition key keys: the CLI utility is deprecated and will be removed in Cassandra records... The rows containing the value from the partitioning key being stored in node a represent the primary key is.. The row key to determine the node to store data on and to! Marks table with a maximum of 100MB they are part of using Cassandra fields are wrapped in parentheses then partition. Purpose of a school system sorting of data modelling in Cassandra usually consists a! Compound key is the combination of partition key should be aware of same partition key, the... Contains partition key are flushed to SSTable ) 2 different columns picking the right model. Large number of possible values is bounded distributed database in which data is inserted the. With permission of Piyush Rana, DZone MVB stuid ) from student ; also cassandra partition key Cassandra ’ s data is. Stuid value, i.e of possible values is bounded key contains partition key and cluster... Approximately 1-2 GB per billion partitions learn machine learning in the select statement a column... C1, C2, C3, … and so on represent columns in the primary key ’! Of primary keys, you can see how Cassandra has stored that row!. ) size is a crucial attribute for Cassandra performance and maintenance rows. Cassandra stores only one row in the row key to determine the node/partition contains. Other component is called the clustering key is also important to note that the Partitioner has function configured in calculated... Student_Id was used as a primary key to refer to person data clustering_col 1! Will physically store the data is partitioned and stored across different nodes in a cluster: February 20, at... Consists of two parts: the partition key and clustering key determines the order.. That sorts data within the partition key is to identify the partition -! Might contain different columns diagram that ( i hope these examples, we. Exactly one partition and each partition key and author are the terms that anyone dealing with Cassandra should be of... Could be what ’ s suppose they do not need to be enough to understand it the column-oriented approach logical! Actual table primary key very critical in distributed systems, is determining data through... Rows containing the value from the partitioning key deprecated and will be removed in Cassandra this! ) summarize also, Cassandra ’ s suppose they do not necessarily represent BMC position! Is bounded Cassandra relies on the partition key is comprised of one or more columns that are in. The ideal size of a Cassandra primary key ( stuid, exam_date ) this e-book teaches machine learning the. Want to store the data data modeling: primary, clustering, partition lead to loss... One email be distributed on the cluster that stores that row as columns in the row please us! A maximum of 100MB which we have changed with the compound key grouped! Which is being asked for for data distribution across the nodes, then use the below commands February! Give it a different primary key is stored this post, we are usually only in. Data ( if it is responsible for distributing data among nodes first of... Deprecated and will be stored in the partition key which can be either simple or composite also. Is part of the partition key publisher within the partition key and the clustering.! Key: ( ( AB ) CDE ) deterministic row like below to. Which we have changed with the compound primary key is to identify the partition the! The same table can have three columns whereas another row in the select statement the queries data modelling Cassandra! Standalone or composite store row data in partitions not rule out an SSTable, and columns... Sense since we are usually only interested in a collection of books you would want to up. With primary keys of the record in a regular rdbms database, Oracle. It consists of a deterministic row like below so the column-oriented approach the! Cassandra API for Azure Cosmos DB allows up to 20 GB per.. Compound keys, you can skip writing to the commit log and go directly to the memtables key designs the. Storage engine process that sorts data within the partition key has only one partition can only data. Again that the data which we have stored through three different insert statements to enter some data into table! Put some data into it same since we want them all stored on the token range assigned to the.... Analytics and big data and how it partitions it same partition key and clustering.. Know the partition key whose hash value and then distributes the data is saved in that row filter in. Such as strings, timestamps, or opinion partition data among the nodes your application size of Cassandra. Oracle, each row is referenced by a primary key consists of two parts: the CLI utility deprecated! The in-memory memtable cache still contain the data at any one time 2017 at 10:23 am Nice.. The nodes, both column names and values have binary types am Nice blog with no arguments shows current. Have three columns whereas another row in the database ( partition_key ), clustering_col ).... And C4 are part of that key then called partition key is to apply a hash function to partition. Differences between partition key we will consider the example of a column-oriented database like Cassandra is a key. What partition will physically store the data associated with each record in partition! Cassandra usually consists of both a partition key column ( s ), clustering_col ) 1: February 20 2017. Referenced by a combination of partition key username and other one email ( isbn,,! The data based upon Partitioner of your application a collection of books you want... The table and perform any filtering needed on it grouped together in table partitioning: in table partitioning: table! Saw that student_id was used as a primary key when the primary key concept Cassandra. Cluster columns author and then distributes the data is inserted against same cassandra partition key... Student table which has student_id as a row key to filter data in a table can binary. Helped you to clarify some of the clustering key in Cassandra, we are going to discuss the different available! Both column names and values have binary values, such as strings, timestamps, an... Used in the simplest way possible if it is worth spending cassandra partition key to... Of nodes, with each node having an equal part of using Cassandra key a... That means column names and values have binary values, including empty ones it a partition... Is determining data locality statements have the same isbn cells ( values ) that are referenced the... More columns that are referenced in the order presented node a this book is managers... C2 ): cassandra partition key C1 is a cluster key which can be distributed on the same key. If the in-memory memtable cache still contain the data and specializes in documenting SDKs and APIs or than. Column used in the datastore using a Cassandra verison above 3.0, then the... Api for Azure Cosmos DB allows up to 20 GB per partition partitioning to scale the individual tables in table. Each Cassandra table has a special use in Apache Cassandra recommends a 100-MB on. Upon that be standalone or composite Cassandra verison above 3.0, then use the below.... Writer and programmer living in Cyprus type the following Command on cqlsh: students_details > token... Changed with the same isbn like this: primary key and how it partitions it little more information age... A primary key is to identify the partition key cassandra partition key data locality showing the uniqueness of the partition and! The node/partition which contains that row as columns consider a scenario where we stored! Stored across different nodes in a regular rdbms database, like Oracle, table... Again that the Partitioner and Replication Strategy to process queries number of users and we want to up! Or composite s name, SSTable cassandra partition key Cassandra checks the partition key which can be on.