Follower Reads

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A follower read is performed on the nearest replica relative to the SQL gateway that is executing the SQL statement regardless of the replica's leaseholder status. Using the nearest replica can reduce read latencies and increase throughput. Applications in multi-region deployments especially can use follower reads to get improved performance.

Follower read types

A strong follower read is a read taken from a Global table. Such tables are optimized for low-latency reads from every region in the database. The tradeoff is that writes will incur higher latencies from any given region, since writes have to be replicated across every region to make the global low-latency reads possible. For more information about global tables, including troubleshooting information, see Global Tables.

A stale follower read is a historical read taken from the nearest replica. You should use stale follower reads only when your application can tolerate reading stale data, since the results of stale follower reads may not reflect the latest writes against the tables you are querying.

The following table summarizes the read types and how to accomplish them.

Stale follower reads

CockroachDB provides the following types of stale follower reads:

• Exact staleness read: A historical read as of a static, user-provided timestamp. See Exact staleness reads.
• Bounded staleness read: A historical read that uses a dynamic, system-determined timestamp to minimize staleness while being more tolerant to replication lag than an exact staleness read. See Bounded staleness reads.

Exact staleness reads

An exact staleness read is a historical read as of a static, user-provided timestamp.

For requirements and limitations, see Exact staleness reads and long-running writes and Exact staleness read timestamps must be far enough in the past.

When to use exact staleness reads

Use exact staleness follower reads when you:

• Need multi-statement reads inside transactions.
• Can tolerate reading older data (at least 4.2 seconds in the past), to reduce the chance that the historical query timestamp is not quite old enough to prevent blocking on a conflicting write and thus being able to be served by a local replica.
• Do not need the increase in availability provided by bounded staleness reads in the face of network partitions or other failures.
• Need a read that is slightly cheaper to perform than a bounded staleness read, because exact staleness reads don't need to dynamically compute the query timestamp.

Run queries that use exact staleness follower reads

Any SELECT statement with an appropriate AS OF SYSTEM TIME value is an exact staleness follower read. You can use the convenience function follower_read_timestamp(), which returns a TIMESTAMP that provides a high probability of being served locally while not blocking on conflicting writes.

Use this function in an AS OF SYSTEM TIME statement as follows:

SELECT ... FROM ... AS OF SYSTEM TIME follower_read_timestamp();

To see the current value of the follower read timestamp, execute the following query:

SELECT now() - follower_read_timestamp();

Exact staleness follower reads demo

The following video describes and demonstrates exact staleness follower reads.

Exact staleness follower reads in read-only transactions

You can set the AS OF SYSTEM TIME clause's value for all operations in a read-only transaction:

BEGIN;

SET TRANSACTION AS OF SYSTEM TIME follower_read_timestamp();
SELECT ...
SELECT ...

COMMIT;

Follower reads are "read-only" operations; you cannot use them in read-write transactions.

Bounded staleness reads

A bounded staleness read is a historical read that uses a dynamic, system-determined timestamp to minimize staleness while being more tolerant to replication lag than an exact staleness read. Bounded staleness reads also help increase system availability, since they provide the ability to serve reads from local replicas even in the presence of network partitions or other failures that prevent the SQL gateway from communicating with the leaseholder.

When to use bounded staleness reads

Use bounded staleness follower reads when you:

• Need minimally stale reads from the nearest replica without blocking on conflicting transactions. This is possible because the historical timestamp is chosen dynamically and the least stale timestamp that can be served locally without blocking is used.
• Can confine the read to a single statement that meets the bounded staleness limitations.
• Need higher availability than is provided by exact staleness reads. Specifically, what we mean by availability in this context is:
  • The ability to serve a read with low latency from a local replica rather than a leaseholder.
  • The ability to serve reads from local replicas even in the presence of a network partition or other failure event that prevents the SQL gateway from communicating with the leaseholder. Once a replica begins serving follower reads at a timestamp, it will always continue to serve follower reads at that timestamp. Even if the replica becomes completely partitioned away from the rest of its range, it will continue to stay available for (increasingly) stale reads.

Run queries that use bounded staleness follower reads

To get a bounded staleness read, use one of the following built-in functions:

This example performs a bounded staleness follower read against a demo cluster with the MovR dataset.

1. Start the demo cluster with 3 nodes:

   cockroach demo --nodes=3
   

2. Issue a single-statement point query to select a single row from a table at a historical timestamp by passing the output of the with_max_staleness() function to the AS OF SYSTEM TIME clause:

   SELECT code FROM promo_codes AS OF SYSTEM TIME with_max_staleness('10s') where code = '0_explain_theory_something';
   

                code
   ------------------------------
     0_explain_theory_something
   (1 row)
   

   The query returns successfully.

   If it had failed with the following error message, you would need to troubleshoot your query to ensure it meets the conditions required for bounded staleness reads.

   ERROR: unimplemented: cannot use bounded staleness for queries that may touch more than one row or require an index join
   SQLSTATE: 0A000
   HINT: You have attempted to use a feature that is not yet implemented.
   See: https://go.crdb.dev/issue-v/67562/v23.2
   

   You can verify using EXPLAIN that the reason this query was able to perform a bounded staleness read is that it performed a point lookup from a single row:

   EXPLAIN SELECT code FROM promo_codes AS OF SYSTEM TIME with_max_staleness('10s') where code = '0_explain_theory_something';
   

                                         info
   --------------------------------------------------------------------------------
     distribution: local
     vectorized: true
   
     • scan
       estimated row count: 1 (0.10% of the table; stats collected 4 minutes ago)
       table: promo_codes@primary
       spans: [/'0_explain_theory_something' - /'0_explain_theory_something']
   (7 rows)
   

Verify that CockroachDB is performing follower reads

To verify that a cluster is performing follower reads, go to the Custom Chart debug page in the DB Console and add the metric follower_reads.success_count to the time-series graph. The number of follower reads performed by your cluster will be shown.

To verify that a specific query uses a follower read, use EXPLAIN ANALYZE to see the statement plan. For example, to test the preceding example:

1. Use the \demo ls shell command to list the connection parameters for all nodes:

   \demo ls
   

   The output will list the connection parameters for each node:

   node 3:
   ...
      (sql)      postgresql://demo:[email protected]:26259/movr?options=-ccluster%3Ddemoapp&sslmode=equire&sslrootcert=%2FUsers%2Fuser%2F.cockroach-demo%2Fca.crt
   

2. Open a new terminal and open the SQL shell on a non-leaseholder node. For example:

   Identify the leaseholder node:

   SELECT lease_holder FROM [SHOW RANGES FROM TABLE promo_codes WITH DETAILS];
   

     lease_holder
   ----------------
                1
   

   Connect to a node other than node 1 (such as node 3, using the preceding output):

   cockroach sql --url='postgresql://demo:[email protected]:26259/movr?options=-ccluster%3Ddemoapp&sslmode=equire&sslrootcert=%2FUsers%2Fuser%2F.cockroach-demo%2Fca.crt'
   

3. Issue the EXPLAIN ANALYZE statement on the non-leaseholder node:

   EXPLAIN ANALYZE SELECT code FROM promo_codes AS OF SYSTEM TIME with_max_staleness('10s') where code = '0_explain_theory_something';
   

                                         info
   --------------------------------------------------------------------------------
     planning time: 5ms
     execution time: 1ms
     distribution: local
     ...
     historical: AS OF SYSTEM TIME 2024-11-19 21:04:00.829742 (bounded staleness)
   
     • scan
       sql nodes: n3
       kv nodes: n3
       regions: us-east1
       used follower read
       ...
       table: promo_codes@promo_codes_pkey
       spans: [/'0_explain_theory_something' - /'0_explain_theory_something']
   

   In the preceding output, used follower read indicates that the read was served by the follower replica. historical: AS OF SYSTEM TIME ... (bounded staleness) shows that it was a historical, bounded staleness read.

How stale follower reads work

Each CockroachDB range tracks a property called its closed timestamp, which means that no new writes can ever be introduced at or below that timestamp. The closed timestamp is advanced continuously on the leaseholder, and lags the current time by some target interval. As the closed timestamp is advanced, notifications are sent to each follower. If a range receives a write at a timestamp less than or equal to its closed timestamp, the write is forced to change its timestamp, which might result in a transaction retry error.

With follower reads, any replica in a range can serve a read for a key as long as the time at which the operation is performed (i.e., the AS OF SYSTEM TIME value) is less than or equal to the range's closed timestamp.

When a gateway node in a cluster receives a request to read a key with a sufficiently old AS OF SYSTEM TIME value, it forwards the request to the closest node that contains a replica of the data—whether it be a follower or the leaseholder.

For further details, see An Epic Read on Follower Reads.

Known limitations

Exact staleness reads and long-running writes

Long-running write transactions will create write intents with a timestamp near when the transaction began. When an exact staleness follower read encounters a write intent, it will often end up in a "transaction wait queue", waiting for the operation to complete; however, this runs counter to the benefit exact staleness reads provide.

To counteract this, you can issue all follower reads in explicit transactions set with HIGH priority:

BEGIN PRIORITY HIGH AS OF SYSTEM TIME follower_read_timestamp();
SELECT ...
SELECT ...
COMMIT;

Exact staleness read timestamps must be far enough in the past

If an exact staleness read is not using an AS OF SYSTEM TIME value far enough in the past, CockroachDB cannot perform a follower read. Instead, the read must access the leaseholder replica. This adds network latency if the leaseholder is not the closest replica to the gateway node. Most users will use the follower_read_timestamp() function to get a timestamp far enough in the past that there is a high probability of getting a follower read.

Bounded staleness read limitations

Bounded staleness reads have the following limitations:

• They must be used in a single-statement (aka implicit) transaction.
• They must read from a single row.
• They must not require an index join. In other words, the index used by the read query must be either a primary index, or some other index that covers the entire query by STORING all columns.

For example, let's look at a read query that cannot be served as a bounded staleness read. We will use a demo cluster, which automatically loads the MovR dataset.

cockroach demo

SELECT code FROM promo_codes AS OF SYSTEM TIME with_max_staleness('10s') LIMIT 1;
ERROR: unimplemented: cannot use bounded staleness for queries that may touch more than one row or require an index join
SQLSTATE: 0A000
HINT: You have attempted to use a feature that is not yet implemented.
See: https://go.crdb.dev/issue-v/67562/v23.2

As noted by the error message, this query cannot be served as a bounded staleness read because in this case it would touch more than one row. Even though we used a LIMIT 1 clause, the query would still have to touch more than one row in order to filter out the additional results.

We can verify that more than one row would be touched by issuing EXPLAIN on the same query, but without the AS OF SYSTEM TIME clause:

EXPLAIN SELECT code FROM promo_codes LIMIT 5;

                                     info
-------------------------------------------------------------------------------
  distribution: full
  vectorized: true

  • scan
    estimated row count: 1 (0.10% of the table; stats collected 1 minute ago)
    table: promo_codes@primary
    spans: LIMITED SCAN
    limit: 1
(8 rows)

The output verifies that this query performs a scan of the primary index on the promo_codes table, which is why it cannot be used for a bounded staleness read.

For an example showing how to successfully perform a bounded staleness read, see Run queries that use bounded staleness follower reads.

See also

• Follower Reads Topology
• Cluster Settings Overview
• Load-Based Splitting
• Network Latency Page

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