Compare-and-swap in Cassandra
From Cassandra cqlsh:
CREATE KEYSPACE IF NOT EXISTS datomic WITH replication = {'class': 'SimpleStrategy', 'replication_factor' : 3};
CREATE TABLE IF NOT EXISTS datomic.datomic
(
id text PRIMARY KEY,
rev bigint,
map text,
val blob
);
Start a 3+ node cassandra cluster. Get Cassandra running locally on 127.0.0.1, 2, 3. CCM is great for this.
mvn clean package
The program
java -jar target/CassandraCAS-0.0.jar {host} {port} raceSessions {nthreads} {msec} {target}
will start nthreads racing to CAS a counter from 0 up to target. Each thread will pause for msec between operations (or pass 0 msec for no pause).
The following example is enough load to force some write retries on my local dev box:
java -jar target/CassandraCAS-0.0.jar 127.0.0.1 9042 create
java -jar target/CassandraCAS-0.0.jar 127.0.0.1 9042 raceSessions 5 0 500
Setup
java -jar target/CassandraCAS-0.0.jar 127.0.0.1 9042 create
Once per client (need to start at about same time or first one can finish before others begin)
java -jar target/CassandraCAS-0.0.jar 127.0.0.1 9042 raceSessions 1 0 5000
- Collect the outputs from one or more clients running CAS stress test into a single file.
- Load the com.datomic.validate-cas namespace in a Clojure REPL. No deps, so vanilla Clojure REPL will do.
- Run validate-file against the output file. If the :valid? key is false, the validator thinks a consistency violation has occurred.
- When a write timeout occurs, the Java driver can be configured to call a
RetryPolicyto allow clients flexibility in error handling. - The
onWriteTimeoutmethod is passed a singleConsistencyLevelargument, described as "the original consistency level of the write that timed out." This looks a bit sketchy since writes can set two kinds of consistency levels (CL and SCL), but the retry handler only gets informed about one. - When a write timeout occurs on a
SCL=SERIALwrite, the consistency level passed to onWriteTimeout isSERIAL. Retrying with the level given results in anInvalidQueryException.
In short, a query that is valid on initial try is deemed invalid on retry. You can usually see this by running with the following settings
java -Dcom.datomic.CassandraCASRetryPolicy=Documented -jar target/CassandraCAS-0.0.jar 127.0.0.1 9042 raceSessions 5 0 500
Setup
mvn clean package
java -jar target/CassandraCAS-0.0.jar 127.0.0.1 9042 create
First use of driver is off main thread: not ok
java -jar target/CassandraCAS-0.0.jar 127.0.0.1 9042 casThread
com.datastax.driver.core.exceptions.NoHostAvailableException: All host(s) tried for query failed (no host was tried)
at com.datastax.driver.core.exceptions.NoHostAvailableException.copy(NoHostAvailableException.java:84)
at com.datastax.driver.core.DefaultResultSetFuture.extractCauseFromExecutionException(DefaultResultSetFuture.java:289)
at com.datastax.driver.core.DefaultResultSetFuture.getUninterruptibly(DefaultResultSetFuture.java:205)
at com.datastax.driver.core.AbstractSession.execute(AbstractSession.java:52)
at com.datomic.CassandraCAS.casRev(CassandraCAS.java:142)
at com.datomic.CassandraCAS$5.run(CassandraCAS.java:263)
at java.lang.Thread.run(Thread.java:745)
Caused by: com.datastax.driver.core.exceptions.NoHostAvailableException: All host(s) tried for query failed (no host was tried)
at com.datastax.driver.core.RequestHandler.sendRequest(RequestHandler.java:107)
at com.datastax.driver.core.SessionManager.execute(SessionManager.java:538)
at com.datastax.driver.core.SessionManager.executeQuery(SessionManager.java:577)
at com.datastax.driver.core.SessionManager.executeAsync(SessionManager.java:119)
First use of driver is on main thread, then CAS from another thread: ok
java -jar target/CassandraCAS-0.0.jar 127.0.0.1 9042 readMainThenCASThread
CAS to 1
First use of driver is on main thread, then another thread touches mbean then does CAS: not ok
java -jar target/CassandraCAS-0.0.jar 127.0.0.1 9042 readMainThenMbeanCASThread
CAS to 2
com.datastax.driver.core.exceptions.NoHostAvailableException: All host(s) tried for query failed (no host was tried)
at com.datastax.driver.core.exceptions.NoHostAvailableException.copy(NoHostAvailableException.java:84)
at com.datastax.driver.core.DefaultResultSetFuture.extractCauseFromExecutionException(DefaultResultSetFuture.java:289)
at com.datastax.driver.core.DefaultResultSetFuture.getUninterruptibly(DefaultResultSetFuture.java:205)
at com.datastax.driver.core.AbstractSession.execute(AbstractSession.java:52)
at com.datomic.CassandraCAS.casRev(CassandraCAS.java:142)
at com.datomic.CassandraCAS$6.run(CassandraCAS.java:278)
at java.lang.Thread.run(Thread.java:745)
Caused by: com.datastax.driver.core.exceptions.NoHostAvailableException: All host(s) tried for query failed (no host was tried)
at com.datastax.driver.core.RequestHandler.sendRequest(RequestHandler.java:107)
at com.datastax.driver.core.SessionManager.execute(SessionManager.java:538)
at com.datastax.driver.core.SessionManager.executeQuery(SessionManager.java:577)
at com.datastax.driver.core.SessionManager.executeAsync(SessionManager.java:119)
... 4 more
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