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Cassandra: A Journey of Upgrade

For the past few couple months, a huge burden on my shoulder had been upgrading our Cassandra cluster from 1.2.6 to 2.1. I’ve been investing a lot of working hours to figure out the solution. Now that it has been done, I feel it is worthwhile to write down the whole experience.

Why Upgrade?

Actually the imperative reason is that we need transaction support in one of our services. And Cassandra 2.0 introduced a new feature called light-weight transaction, although it is light-weight, it somehow can fix our issue.

Besides that, there are also a couple of new features we can benefit from the upgrade:

Infrastructure

Upgrade Path

Driver upgrade

We’re using a fairly old driver Cassandra called Pycassa, which is no longer maintained. And it is based on thrift protocol, which is deprecated/ditched in the version 3, so all the new and good stuff on the native protocol has nothing to do with Pycassa. Very naturally we switched to the recommended/official driver maintained by the Datastax.

Internally we don’t have a layer for Cassandra, so refactoring is a lot of pain. We have to replace all the code usages of Pycassa among all services, and carefully update all unit tests.

We also bumped into some issues when deploying with the new driver:

The driver upgrade is not as smooth as I thought. A lot of back and forth happened and it took us almost two month or so to ship the upgrade.

No rolling upgrade?

Rolling upgrade should be a default option for a cluster upgrade. But unfortunately it is not supported between major versions of Cassandra. As it is documented here. We thought about workarounds. Like building a new Cluster and syncing data between two clusters. But building a new cluster is not our option due to some “policy”, so we decided that we can tolerate some downtime, and that also means we will update each Cassandra instance in place.

Data backup and restore

It’s important to have a backup of the data. In case something goes wrong, we can go back to the save point. When doing data backup, we demand that all services that access Cassandra should be stopped and keep data untouched during the process.

Below is a typical structure of one of our Cassandra nodes:

/mnt/cassandra/

── commitlog_directory

── data_file_directories

“data_file_directories” is where Cassandra data files live, our goal is to backup this directory. We’ll do a ‘nodetool drain’ on the node, which will flush all memtables to data files. After that We’ll pack data_file_directories into one tarball and upload it to the cloud(to prevent disk failure of node). So we’ll have two copies of data.

Procedure:

If something goes wrong and we want to abort the upgrade and go back to the old version. We simply retrieve the old data and unpack it to the Cassandra data file directory.

The backup and restore procedure are automated by Ansible scripts.

Upgrade

Upgrade directly from current version 1.2.6 to 2.1 is not possible. Since pre-2.0 SSTables are not supported by 2.1. A direct upgrade to 2.1, Cassandra would fail to start and following error would be raised:

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java.lang.RuntimeException: Incompatible SSTable found. Current version ka is unable to read file: /var/lib/cassandra/data/system/schema_keyspaces/system-schema_keyspaces-ic-1. Please run upgradesstables.
        at org.apache.cassandra.db.ColumnFamilyStore.createColumnFamilyStore(ColumnFamilyStore.java:443) ~[apache-cassandra-2.1.1.jar:2.1.1]
        at org.apache.cassandra.db.ColumnFamilyStore.createColumnFamilyStore(ColumnFamilyStore.java:420) ~[apache-cassandra-2.1.1.jar:2.1.1]
        at org.apache.cassandra.db.Keyspace.initCf(Keyspace.java:327) ~[apache-cassandra-2.1.1.jar:2.1.1]
        at org.apache.cassandra.db.Keyspace.<init>(Keyspace.java:280) ~[apache-cassandra-2.1.1.jar:2.1.1]
        at org.apache.cassandra.db.Keyspace.open(Keyspace.java:122) ~[apache-cassandra-2.1.1.jar:2.1.1]
        at org.apache.cassandra.db.Keyspace.open(Keyspace.java:99) ~[apache-cassandra-2.1.1.jar:2.1.1]
        at org.apache.cassandra.db.SystemKeyspace.checkHealth(SystemKeyspace.java:558) ~[apache-cassandra-2.1.1.jar:2.1.1]
        at org.apache.cassandra.service.CassandraDaemon.setup(CassandraDaemon.java:214) [apache-cassandra-2.1.1.jar:2.1.1]
        at org.apache.cassandra.service.CassandraDaemon.activate(CassandraDaemon.java:443) [apache-cassandra-2.1.1.jar:2.1.1]
        at org.apache.cassandra.service.CassandraDaemon.main(CassandraDaemon.java:532) [apache-cassandra-2.1.1.jar:2.1.1]

So we upgraded to 2.0.0 and run upgradesstables command to upgrade SSTables. After that, we then upgrade from 2.0.0 to 2.1.

Cassandra has an internal version for SSTables. During the upgrade, sstable version will be bumping from:

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ic (1.2.6) --> ja (2.0.0) --> ka(2.1.3)

Procedure:

The procedure looks simple and clear. While we had couple issue when doing test upgrade:

Data consistency

How to ensure data are not corrupted during the upgrade? I think this should be guaranteed by Cassandra. However when doing upgrade testing, we have a script to dump all Cassandra data before and after the upgrade to ensure data are not touched. This step is taken away when we’re doing actual upgrade.