44 lines
1.9 KiB
Markdown
44 lines
1.9 KiB
Markdown
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# Distributed addressing
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In distributed Memgraph a single graph element must be owned by exactly
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one worker. It is possible that multiple workers have cached copies of
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a single graph element (which is inevitable), but there is only one
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owner.
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The owner of a graph element can change. This is not yet implemented,
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but is intended. Graph partitioning is intended to be dynamic.
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Graph elements refer to other graph elements that are possibly on some
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other worker. Even though each graph element is identified with a unique
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ID, that ID does not contain the information about where that element
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currently resides (which worker is the owner).
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Thus we introduce the concept of a global address. It indicates both
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which graph element is referred to (it's global ID), and where it
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resides. Semantically it's a pair of two elements, but for efficiency
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it's stored in 64 bits.
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The global address is efficient for usage in a cluster: it indicates
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where something can be found. However, finding a graph element based on
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it's ID is still not a free operation (in the current implementation
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it's a skiplist lookup). So, whenever possible, it's better to use local
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addresses (pointers).
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Succinctly, the requirements for addressing are:
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- global addressing containing location info
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- fast local addressing
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- storage of both types in the same location efficiently
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- translation between the two
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The `storage::Address` class handles the enumerated storage
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requirements. It stores either a local or global address in the size of
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a local pointer (typically 8 bytes).
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Conversion between the two is done in multiple places. The general
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approach is to use local addresses (when possible) only for local
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in-memory handling. All the communication and persistence uses global
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addresses. Also, when receiving address from another worker, attempt to
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localize addresses as soon as possible, so that least code has to worry
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about potential inefficiency of using a global address for a local graph
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element.
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