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What is this?

This is a fork of the spymemcached java client maintained by Shortcut for use in Datomic on-prem peers and transactors.

We maintain a patchset that we apply to the spymemcached library. We then replace their spymemcached jars in the transactor and peers with this one.

Datomic <= 1.0.6316 used https://github.com/couchbase/spymemcached v2.12.3 and the branch based on that is master+patches-on-couchbase-spymemcached and releases tagged like 2.12.3.chX or 2.12.3.scX.

Datomic now appears to use AWS's fork of it that adds ElastiCache auto-discovery https://github.com/awslabs/aws-elasticache-cluster-client-memcached-for-java.

We have a patchset for that on master+patches-on-aws-elasticache but it is not functional. It is based on https://github.com/awslabs/aws-elasticache-cluster-client-memcached-for-java/releases/tag/1.1.3 and our patchset is on branch and releases tagged like 1.1.3.scX.

Cognitect appears to have added to their fork at least an additional configurable timeout value for the ElastiCache-specific "config get " command, and without those patches this release will not work with Datomic.

However, Datomic appears to have some reflection in place to determine if the spymemcached client lib supports AWS discovery. So if we use a different spymemcached library not based on the AWS fork, memcached caching with Datomic still works but doesn't support auto-discovery.

This is fine for us, because we don't need this: we proxy through McRouter. The thing we need is overriding the protocol format to TEXT. (More on this later.)

The patchset includes:

  1. A fix for an NPE thrown from fillWriteBuffer. couchbase#38
  2. A fix for incoherent connection state when a Server responds before the client is finished sending: https://github.com/useshortcut/spymemcached/commit/74838cc0a1107676d6224bdee3abad0c03aa848a
  3. Some additional logging to discover and monitor the above two issues:
  4. Some minor build.xml bitrot cleanup, changelog suppression, and targets Java 11.
  5. Some system properties to override protocol and some timeout values. #2
  6. A package rename from net.spy.memcached to datomic.spy.memcached. This is needed on datomic versions >= 1.0.6726 because of shading.
  7. Removal of the pom.xml file, since it's a development-only tool that we do not need.

The last item requires explanation. We use McRouter (our fork) as a memcached proxy between Datomic peers and multiple ElastiCache clusters in separate AZs. We do this to avoid cross-AZ traffic between the peer and the cluster (saving enough money for more clusters!) and to provide failover redundancy if a cluster goes down. (It's also a flexible platform for more elaborate cache strategies or additional tiers, but we haven't done that.)

However, McRouter only understands the Memcached text protocol; Datomic constructs a memcached client that uses the binary protocol in code we do not control. The system properties are a hack to let us override Datomic's builder choices so we can force use of the text protocol.

This fork adds the following system properties:

  • shortcut.spymemcached.forceProtocol=TEXT (or BINARY)
  • shortcut.spymemcached.forceOpQueueMaxBlockTimeMs How long to wait for an operation to be enqueued.
  • shortcut.spymemcached.forceOpTimeoutMs How long to wait for an operation to complete. This timer begins when the op is created, so it includes the enqueing time mentioned by the previous property. (We think Datomic may set this timeout to 50.)

Building for Shortcut

We only use part of the build system: all we need is a jar and to run tests. ${version} is the result of git describe --abbrev=0.

  1. Build with ant jar. This places a file in build/jars/spymemcached-${version}.jar
  2. Run the integration tests ant test
  3. Run the integration tests with a server using the spymemcached-release-tools.sh script in the backend repo. This includes a stress test designed to trigger the fillWriteBuffer NPE.

For more thorough building, testing, and deployment docs see the datomic guide.

Rebasing on upstream changes

  1. Rebase, reapply patches, and check for dependency changes.
  2. Determine the new version number. It should be {upstream-version}.sc1. Increment sc1 (sc2, etc) only if our build changed but upstream did not.
  3. Bump filename and version of spymemcached pom in root and change dependencies as needed. Commit locally.
  4. Temporarily tag the new version with an annotated git tag, e.g. git tag 2.12.3.sc5 -m "Shortcut release 5 of Couchbase memcached 2.12.3" Do not push this tag! The build system uses git tags for versioning.
  5. Then build, test and deploy artifact as above.
  6. Then put the artifact in our local repo, adjust versions, and test with our applications.
  7. PR your changes to this fork.
  8. After review and merge (merge-commit please!), delete the tag from your local repository, create the same annotated tag on master, and push it.

Text below this line is from upstream and kept unchanged.

Building

Spymemcached can be compiled using Apache Ant by running the following command:

ant

This will generate binary, source, and javadoc jars in the build directory of the project.

To run the Spymemcached tests against Membase Server run the following command:

ant test -Dserver.type=membase

To test Spymemcached against Membase running on a different host use the following command:

ant test -Dserver.type=membase \
    -Dserver.address_v4=ip_address_of_membase

Testing

The latest version of spymemcached has a set of command line arguments that can be used to configure the location of your testing server. The arguments are listed below.

-Dserver.address_v4=ipv4_address_of_testing_server

This argument is used to specify the ipv4 address of your testing server. By default it is set to localhost.

-Dserver.address_v6=ipv6_address_of_testing_server

This argument is used to set the ipv6 address of your testing server. By default it is set to ::1. If an ipv6 address is specified then an ipv4 address must be specified otherwise there may be test failures.

-Dserver.port_number=port_number_of_memcached

This argument is used when memcahched is started on a port other than 11211

-Dtest.type=ci

This argument is used for CI testing where certain unit tests might be temporarily failing.

More Information

For more information about Spymemcached see the links below:

Project Page The

Spymemcached Project Home contains a wiki, issue tracker, and downloads section.

Github

The gitub page contains the latest Spymemcached source.

Couchbase.org

At couchbase.org you can find a download's section for the latest release as well as an extensive tutorial to help new users learn how to use Spymemcached.