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https://issues.apache.org/jira/browse/IGNITE-640?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15175427#comment-15175427
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Yakov Zhdanov commented on IGNITE-640:
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Hi Konstantin!
Very good start!
I have reviewed
https://github.com/apache/ignite/compare/master...ruskim:ignite-640?expand=1
and here are my comments.
1. I don't think we need busy lock in the map. I don't see in in IgniteSet or
IgniteQueue either. Can you please point me to the usages in data structures?
2. IgniteMultimap interface should be documented and contain "public" modifiers
for members. Please take a look at IgniteQueue, for instance, and be consistent.
3. I don't like ArrayLists as values. They will often have 1-2-3 elements. I
would use exact size arrays. This way we avoid unnecessary memory overhead. Or
investigate use single-linked list structure. Maybe we should provide an
ability to choose the backing structure. I.e. if map is not supposed to change
very often then arrays seems to be perfect.
4. Using "key" provided by user as map key is not correct. You can end up with
the situation when several multimaps can backed by the same cache. This way you
require keys to be unique across all maps. Please see GridCacheQueueItemKey or
GridCacheSetItemKey
5. Can I ask you to fully design multimap interface and then send notice to dev
list so others can review?
6. I would also think of providing support for comparable values.
Yakov
> Implement IgniteMultimap data structures
> ----------------------------------------
>
> Key: IGNITE-640
> URL: https://issues.apache.org/jira/browse/IGNITE-640
> Project: Ignite
> Issue Type: Sub-task
> Components: data structures
> Reporter: Dmitriy Setrakyan
> Assignee: Konstantin Margorin
>
> We need to add {{IgniteMultimap}} data structure in addition to other data
> structures provided by Ignite. {{IgniteMultiMap}} should have similar API to
> {{java.util.Map}} class in JDK, but support the semantics of multiple values
> per key, similar to [Guava
> Multimap|http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/collect/Multimap.html].
>
> However, unlike in Guava, our multi-map should work with Lists, not
> Collections. Lists should make it possible to support the following methods:
> {code}
> // Gets value at a certain index for a key.
> V get(K, index);
> // Gets all values for a collection of keys at a certain index.
> Map<K, V> getAll(Collection<K>, index);
> // Gets values for specified indexes for a key.
> List<V> get(K, Iterable<Integer> indexes);
> // Gets all values for a collection of keys at specified indexes.
> Map<K, Collection<V>> getAll(Collection<K>, Iterable<Integer> indexes);
> // Gets values for specified range of indexes, between min and max.
> List<V> get(K, int min, int max);
> // Gets all values for a collection of keys for a specified index range,
> between min and max.
> Map<K, Collection<V>> getAll(Collection<K>, int min, int max);
> // Gets all values for a specific key.
> List<V> get(K);
> // Gets all values for a collection of keys.
> Map<K, List<V>> getAll(Collection<K>);
> // Iterate through all elements with a certain index.
> Iterator<Map.Entry<K, V>> iterate(int idx);
> // Do we need this?
> Collection<IgniteTuple<Integer V>> get(K, IgniteBiPredicate<Integer, V>)
> {code}
> Multimap should also support colocated and non-colocated modes, similar to
> [IgniteQueue|https://github.com/apache/incubator-ignite/blob/master/modules/core/src/main/java/org/apache/ignite/IgniteQueue.java]
> and its implementation,
> [GridAtomicCacheQueueImpl|https://github.com/apache/incubator-ignite/blob/master/modules/core/src/main/java/org/apache/ignite/internal/processors/datastructures/GridAtomicCacheQueueImpl.java].
> h2. Design Details
> The most natural way to implement such map, would be to store every value
> under a separate key in an Ignite cache. For example, let's say that we have
> a key {{K}} with multiple values: {{V0, V1, V2, ...}}. Then the cache should
> end up with the following values {{K0, V0}}, {{K1, V1}}, {{K2, V2}}, etc.
> This means that we need to wrap user key into our own, internal key, which
> will also have {{index}} field.
> Also note that we need to collocate all the values for the same key on the
> same node, which means that we need to define user key K as the affinity key,
> like so:
> {code}
> class MultiKey<K> {
> @CacheAffinityMapped
> private K key;
> int index;
> }
> {code}
> Look ups of values at specific indexes becomes very simple. Just attach a
> specific index to a key and do a cache lookup. Look ups for all values for a
> key should work as following:
> {code}
> MultiKey key;
> V v = null;
> int index = 0;
> List<V> res = new LinkedList<>();
> do {
> v = cache.get(MultiKey(K, index));
> if (v != null)
> res.add(v);
> index++;
> }
> while (v != null);
> return res;
> {code}
> We could also use batching for performance reason. In this case the batch
> size should be configurable.
> {code}
> int index = 0;
> List<V> res = new LinkedList<>();
> while (true) {
> List<Key> batch = new ArrayList<>(batchSize);
> // Populate batch.
> for (; index < batchSize; index++)
> batch.add(new MultiKey(K, index % batchSize);
> Map<Key, V> batchRes = cache.getAll(batch);
> // Potentially need to properly sort values, based on the key order,
> // if the returning map does not do it automatically.
> res.addAll(batchRes.values());
> if (res.size() < batch.size())
> break;
> }
> return res;
> {code}
> h2. Evictions
> Evictions in the {{IgniteMultiMap}} should have 2 levels: maximum number of
> keys, and maximum number of values for a key. The maximum number of keys
> should be controlled by Ignite standard eviction policy. The maximum number
> of values for a key should be controlled by the implementation of the
> multi-map. Either eviction parameter should be configurable.
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