KairosDB is less popular, smaller, but with different limitations and is more flexible. It was inspired by the OpenTSDB design but then took a different path.
OpenTSDB relies on HBase, KairosDB has configurable and pluggable datastore - but the only production-ready so far is using Cassandra.
OpenTSDB always does interpolation of values for aggregation (which I found to be an hazardous decision), KairosDB does not really do proper series "vertical" aggregation (by vertical I mean not downsampling).
OpenTSDB is GPL, KairosDB is Apache 2.0 (that counts for closed-source integrations).
OpenTSDB supports only numerical data but supports annotations, KairosDB supports Strings and numerical in baseline but is compatible with any data type, it does not have annotations (but you may use string for that).
On their baseline OpenTSDB produces graphs on the server, KairosDB produces graphs on the client.
Both are integrated with Grafana time series dashboard, OpenTSDB has more side projects, KairosDB is the only time series database I know for being integrated with a reporting tool (BIRT).
OpenTSDB requires to create the metrics in advance using a special tool (needs to lock the cluster to allocate a new ID), KairosDB can have any kind of new metric on the fly.
If you need something modular for building custom features I strongly recommend KairosDB. Look at the code, it's really nicely crafted.
Otherwise, they both have goods, I found that KairosDB is also much less limited on the cornersides (while having less side-projects), and we now use kairosDB intensively.
OpenTSDB relies on HBase, KairosDB has configurable and pluggable datastore - but the only production-ready so far is using Cassandra.
OpenTSDB always does interpolation of values for aggregation (which I found to be an hazardous decision), KairosDB does not really do proper series "vertical" aggregation (by vertical I mean not downsampling).
OpenTSDB is GPL, KairosDB is Apache 2.0 (that counts for closed-source integrations).
OpenTSDB supports only numerical data but supports annotations, KairosDB supports Strings and numerical in baseline but is compatible with any data type, it does not have annotations (but you may use string for that).
On their baseline OpenTSDB produces graphs on the server, KairosDB produces graphs on the client.
Both are integrated with Grafana time series dashboard, OpenTSDB has more side projects, KairosDB is the only time series database I know for being integrated with a reporting tool (BIRT).
OpenTSDB requires to create the metrics in advance using a special tool (needs to lock the cluster to allocate a new ID), KairosDB can have any kind of new metric on the fly.
If you need something modular for building custom features I strongly recommend KairosDB. Look at the code, it's really nicely crafted.
Otherwise, they both have goods, I found that KairosDB is also much less limited on the cornersides (while having less side-projects), and we now use kairosDB intensively.