Friday, April 13, 2012

Coherence 101, few things about read through you may want to know

Read-through is a technique which allows cache to automatically populate entry for external data source up on cache miss. Oracle Coherence supports this technique via read-write-backing-map and application provided cache loaders (you can read more in Coherence documentation).

CacheLoader/CacheStore vs. BinaryEntryStore

You cache loader/store plug-in may either implement CacheLoader/CacheStore interface or BinaryEntryStore interface. BinaryEntryStore have following key advantages:

  • It can work with binary objects, which allows you to avoid unneeded serialization/deserialziation in some case.
  • It is possible to distinguish inserts vs. updates using BinaryEntryStore. BinaryEntryinterface provides you access to both new and previous version of value, this may be very useful.
  • Why Coherence is doing load() before store()?

    Assume that we working with key which does not exist in cache. If you just put(…) new key via named cache interface, Coherence would work as expected. It will add object to a cache and call store(…) in cache store plug-in. But if you will use entry processor and setValue(…) for entry which is not in cache – surprise, surprise – Coherence will first load(…) key and then store(…) new value.
    Reason is simple, setValue(…) should return pervious value as result of operation. Use other version of method – setValue(value, false) to avoid unnecessary load(…) call. BTW way putAll(…) should be preferred over put(…) for same reason – putAll(…) is not required to return previous value.

    load() vs. loadAll() methods

    Assume that your cache loader using SQL to fetch data from RDBMS. It is clear what single SQL select retrieving N entries (e.g. using in (…) in where clause) at once is better than N subsequent SQL selects each fetching only one entry.
    Prior to Coherence 3.7, read-write backing map implementation were using sequential approach (making bulk cache preloading with read-though impractical). In Coherence 3.7 this was fixed (but you should use at least 3.7.1.3 version, earlier versions have known bugs related to read-through).
    So, in 3.7 getAll() will use loadAll() under hood (but remember that your key set will be split to partitions, distributed across storage members and each storage member will process read-though in partition-parallel fashion).
    But will it work with aggregators and entry processors invoked over collection of keys? – not so fast …
    BTW If you stick with 3.6 or earlier you can read about work around here.

    Aggregator warm up

    Assume that you know key set you want to aggregate using Coherence distributed aggregation, but some many of these keys may not be in cache (i.g. not-yet-loaded). Read-though is enabled.
    Instance of your aggregator started on storage node will receive set of BinaryEntrys from Coherence. But it does mean that all these entries are present in cache, Coherence will not try to preload working set for aggregator. Actually aggregator may decide to ignore data-not-in-cache (see isPresent() method). But if it call any kind of “get” methods on entry, Coherence will load value via cache loader plug-in. Problem is – it will be done in sequential manner, so this may take A LOT of time.
    Can we work this around? - Sure.
    Simplest workaround is call getAll() before invoking aggregator (but it kills idea of distributed aggregation). A smarter way is dig though internal cache layers and load entries via call to read-write-backing-map. Snippet below can be used for effective preloading for set of entries in aggregators and entry processors.
    public static void preloadValuesViaReadThrough(Set<BinaryEntry> entries) {
     CacheMap backingMap = null;
     Set<Object> keys = new HashSet<Object>();
     for (BinaryEntry entry : entries) {
      if (backingMap == null) {
       backingMap = (CacheMap) entry.getBackingMapContext().getBackingMap();
      }
      if (!entry.isPresent()) {
       keys.add(entry.getBinaryKey());
      }
     }
     backingMap.getAll(keys);
    }
    

    Aggregation, expiry and past expiry entry resurrection

    If you are using read-write backing map in combination with expiry, you may be prone to following effect.
     Assume that your cache is idle for some time and some of cache entries are already past their expiry. Now you are issuing an aggregator over all cache data (in my case it was a regular housekeeping job interested only in live cache data). Filters in Coherence can match only cache data (they never trigger read-though), but surprisingly, operation described above starts storming DB with read-through requests!
    What has happen?
    Lazy expiry
    Local cache (acting as internal map for read-write backing map) is doing expiry passively. If you are not touching it, it cannot expire anything. But if you call any of its method, expiry check will be triggered and entries may be physically removed for cache at this point.
    Key index of partitioned cache service
    Partitioned cache service has internal structure called “key index” – it is simply a set of all keys in local backing map. When you issuing a filter based operation, Coherence calculates key set first (using filter), then perform operation (e.g. aggregation) over know set of keys. A set of all keys are passed to filter, then filter may decide which keys to process (it can consult with indexes at this point) and whenever further filtering by value is required. AlwaysFilter is very simple; it does not require any value filtering, so Coherence just passing whole “key index” content as input for aggregation without consulting with backing map.
    Together
    A lot of entries in cache are past expiry, but they are still in cache because it is idle and local cache has no opportunity to perform expiry check. Aggregator with AlwaysFilter is issued, and Coherence storage member will perform aggregation against all keys currently in “key index” (including key past their expiry). Access to first entry from aggregator will trigger expiry check in backing map, effectively wiping out expired entries. But aggregator instance is already started and its entry set already has these keys. By processing recently expired entries, which are in its entry set, aggregator will be triggering read-though resurrecting them (and of cause it would be doing it one by one – read SLOW).
    How to prevent this?
    Well, my conditions are little exotics. You probably never hit exactly this problem, but still understanding of such effects may be helpful for related cases.
    Workaround is dead simple – call size() on cache just before issuing an aggregator. size() will hit backing map, it will have a chance to process expiry, and by the moment of aggregator arrival dead entries will be removed from “key index” thus no unexpected read-though would happen.

    Conclusion

    Live is full of surprises when it comes to complex distributed systems. Keep your eyes open ;)

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