An overview of Neo4j Internals

An overview of Neo4j Internals tobias@neotechnology.com Tobias Lindaaker twitter: @thobe, #neo4j (@neo4j) web: neo4j.org neotechnology.com Hacker @ Neo Technology my web: thobe.orgMonday, May 21, 2012

Outline This is a rough structure of how the pieces of Neo4j fit together. This talk will not cover how disks/fs works, we just assume it does. Traversals Core API Cypher Nor will it cover the “Core API”, you are assumed to know it. Node/Relationship Thread local diffs Object cache FS Cache HA Record ﬁles Transaction log Disk(s) 2Monday, May 21, 2012

Outline Traversals Core API Cypher Node/Relationship Thread local diffs Object cache FS Cache HA Let’s start at the bottom: the on disk Record ﬁles Transaction log storage file layout. Disk(s) 3Monday, May 21, 2012

Simple sample graph. It all boils down to linked lists of fixed size records on disk. Your graph on disk Properties are stored as a linked list of property records, each holding key+value. Each node/relationship references its first property record. The Nodes also reference the first node in its relationship chain. Each Relationship references its start and Name: Alistair end node. Age: 34 KNOWS It also references the prev/next relationship record for the start/end node respectively Name: Tobias Age: 27 Nationality: Swedish KNOWS KNOWS KNOWS Name: Ian Age: 42 Name: Jim Age: 37 KNOWS Stuff: good 4Monday, May 21, 2012

Simple sample graph. It all boils down to linked lists of fixed size records on disk. Your graph on disk Properties are stored as a linked list of property records, each holding key+value. Each node/relationship references its first Name property record. The Nodes also reference the first node in its Alistair relationship chain. Each Relationship references its start and end node. Name KNOWS It also references the prev/next relationship record for the start/end node respectively Tobias Age 34 Age 27 Nationality KNOWS Swedish KNOWS KNOWS Name Jim Age Name 37 Stuff Ian KNOWS Age good 42 4Monday, May 21, 2012

Simple sample graph. It all boils down to linked lists of fixed size records on disk. Your graph on disk Properties are stored as a linked list of property records, each holding key+value. Each node/relationship references its first Name SP EP property record. The Nodes also reference the first node in its Alistair relationship chain. SN EN Each Relationship references its start and end node. Name KNOWS It also references the prev/next relationship record for the start/end node respectively Tobias Age 34 Age 27 SP EP SP EP SN EN Nationality SN EN KNOWS Swedish KNOWS SP EP SN EN KNOWS Name SP EP Jim Age SN EN Name 37 Stuff Ian KNOWS Age good 42 4Monday, May 21, 2012

Store ﬁles ๏Node store ๏Relationship store • Relationship type store ๏Property store • Property key store • (long) String store Short string and array values are inlined in the • property store, long values are stored in (long) Array store separate store files. 5Monday, May 21, 2012

Neo4j Storage Record LayoutNode (9 bytes)inUse nextRelId nextPropId 1 5 9Relationship (33 bytes)inUse firstNode secondNode relationshipType firstPrevRelId firstNextRelId secondPrevRelId secondNextRelId nextPropId 1 5 9 13 17 21 25 29 33Relationship Type (5 bytes)inUse typeBlockId 1 5Property (33 bytes)inUse type keyIndexId propBlock nextPropId 1 3 5 29 33Property Index (9 bytes)inUse propCount keyBlockId 1 5 9Dynamic Store (125 bytes)inUse next data 1 5NeoStore (5 bytes)inUse datum 1 5Monday, May 21, 2012

Outline Traversals Core API Cypher Next: The t wo levels of cache Node/Relationship in Neo4j. Thread local diffs The low level FS Cache for the Object cache record files. And the high level Object cache storing a structure more optimized for traversal. FS Cache HA Record ﬁles Transaction log Disk(s) 7Monday, May 21, 2012

The caches ๏ Filesystem cache: • Caches regions store file intofiles sized regions) (divides each of the store equally • The cache holds a fixed number of regions for each file • Regions are evicted based on ahit in non-cached region) (hit count vs. miss count, i.e. LFU-like policy • Default implementation of regions uses OS mmap ๏ Node/Relationship cache • Cache a version more optimized for traversals 8Monday, May 21, 2012

What we put in cache ID Relationship ID refs in: R1 R2 ... Rn The structure of the elements in the high level type 1 object cache. out R1 R2 ... Rn On disk most of the information is contained in: R1 R2 R3 ... Rn in the relationship records, with the nodes just type 2 referencing their first relationship. In the out R1 ... Rn Node cache this is turned around: the nodes hold references to all its relationships. The ... (grouped by type) relationships are simple, only holding its properties. The relationships for each node is grouped by RelationshipType to allow fast traversal of a specific type. Key 1 Key 2 ... Key n All references (dotted arrows) are by ID, and traversals do indirect lookup through the cache. Val 1 Val 2 ID start end type Val n Relationship Key 1 Key 2 ... Key n Val 1 Val 2 Val n 9Monday, May 21, 2012

Outline So how do traversals work... Traversals Core API Cypher Node/Relationship Thread local diffs Object cache FS Cache HA Record ﬁles Transaction log Disk(s) 10Monday, May 21, 2012

Traversals - how do they work? ๏ RelationshipExpanders: given (a path to) a node, returns The surface layer, the you Relationships to continue traversing from that node interact with. ๏ Evaluators: given (a path to) a node, returns whether to: • Continue traversing on that branch (i.e. expand) or not • Include (the path to) the node in the result set or not ๏ Then a projection to Path, Node, or Relationship applied to each Path in the result set. ... but also: ๏ Uniqueness level: policy for when it is ok to revisit a node that has already been visited ๏ Implemented on top of the Core API 11Monday, May 21, 2012

More on Traversals ๏ Fetch node data from cache - non-blocking access This is what happens • under the hood. If not in cache, retrieve from storage, into cache ‣If region is in FS cache: blocking but short duration access ‣If region is outside FS cache: blocking slower access ๏ Get relationships from cached node • If not fetched, retrieve from storage, by following chains ๏ Expand relationship(s) to end up on next node(s) • The relationship knows the node, no need to fetch it yet ๏ Evaluate • possibly emitting a Path into the result set ๏ Repeat 12Monday, May 21, 2012

Outline How is Cypher different? Traversals Core API Cypher and how dowes it work? Node/Relationship Thread local diffs Object cache FS Cache HA Record ﬁles Transaction log Disk(s) 13Monday, May 21, 2012

Cypher - Just convenient traversal descriptions? ๏ Builds on the same infrastructure as Traversals - Expanders • but not on the full Traversal system ๏ Uses graph pattern matching for traversing the graph • Recursive MATCH x-->y, backtracking z-->a-->b, z-->b START x=... matching with x-->z, y-->z, Red: pattern graph Blue: actual graph Green: start node Purple: matches 14Monday, May 21, 2012

What about gremlin? ๏ gremlin is a third party language, built by Marko Rodriguez of Tinkerpop (a group of people who like to hack on graphs) ๏ Originally based on the idea of using xpath to describe traversals: ./HAS_CART/CONTAINS_ITEM/PURCHASED/PURCHASED but bastardized to distinguish between nodes and relationships: ./outE[label=HAS_CART]/inV Traversals are close /outE[label=CONTAINS_ITEM]/inV to xpath, which is why xpath like /inE[label=PURCHASED]/outV descriptions of traversals seemed /outE[label=PURCHASED]/inV like a good idea. ๏ xpath is not complete enough to express full algorithms, it needs a host language, gremlin originally deﬁned its own. This changed Groovy as a more complete host language and abandoned xpath in favor of method chaining [ replace ‘/’ with ‘.’ ] 15Monday, May 21, 2012

Gremlin compared to Cypher ๏start me=node:people(name={myname}) match me-[:HAS_CART]->cart-[:CONTAINS_ITEM]->item item<-[:PURCHASED]-user-[:PURCHASED]->recommendation return recommendation ๏ Cypher is declarative, describes what data to get - its shape ๏ Gremlin is imperative, prescribes how to get the data ๏ Cypher has more opportunities for optimization by the engine ๏ Gremlin can implement pagerank, Cypher can’t (yet?) 16Monday, May 21, 2012

Outline Traversals Core API Cypher Transactions involve t wo parts: The (thread local) changes being done by an active transaction, Node/Relationship and the transaction replay log Thread local diffs Object cache for recovery. FS Cache HA Record ﬁles Transaction log Disk(s) 17Monday, May 21, 2012

Transaction Isolation ๏ Mutating operations are not written when performed ๏ They are stored in a thread conﬁned transaction state object ๏ This prevents other threads from seeing uncommitted changes from the transactions of other threads ๏ When Transaction.ﬁnish() is invoked the transaction is either committed or rolled back ๏ Rollback is simple: discard the transaction state object 18Monday, May 21, 2012

Transactions & Durability ๏ Commit is: • Changes made in the transaction are collected as commands • Commands are sorted to get predictable update order ‣This prevents concurrent readers from seeing inconsistent data when the changes are applied to the store • Write changes (in sorted order) to the transaction log • Mark the transaction as committed in the log • Apply the changes (in sorted order) to the store ﬁles 19Monday, May 21, 2012

Recovery ๏ Transaction commands dictate state, they don’t modify state • i.e. SET property "count" to 5 • rather than ADD 1 to property "count" ๏ Thus: Applying the same command twice yields the same state ๏ Recovery simply replays all transactions since the last safe point ๏ If tx A mutates node1.name, then tx B also mutates node1.name that doesn’t matter, because the database is not recovered until all transactions have been replayed 20Monday, May 21, 2012

Outline Traversals Core API Cypher Node/Relationship Thread local diffs Object cache FS Cache HA Record ﬁles Transaction log High Availability in Neo4j builds on top of the transaction replay Disk(s) 21Monday, May 21, 2012

Outline The transaction logs are shared bet ween all instances in an High Availability setup, all other parts operate on the local data just like in the standalone case. Traversals Core API Cypher Local Node/Relationship Thread local diffs Object cache FS Cache HA Record ﬁles Transaction log Shared Disk(s) 22Monday, May 21, 2012

• HA - the parts to it: ๏ Based on streaming transactions between servers ๏ All transactions are committed through the master • Then (eventually) applied to the slaves • Eventuality synchronizationupdate intervalby interaction or when deﬁned by the is mandated ๏ When writing to a slave: • Locks coordinated through the master • Transaction data buffered on theget a txid applied ﬁrst on the master to slave then applied with the same txid on the slave 23Monday, May 21, 2012

Creating new Nodes and Relationships ๏ New Nodes/Relationships don’t need locks, so they don’t need a transaction synced with master until the transaction commit ๏ They do need an ID that is unique and equal among all instances ๏ Each instance allocates IDs in blocks from the master, then assigns them to new Nodes/Relationships locally • This batch allocation can be seen in (Enterprise) 1000 as Node/Relationship counts jumping in steps of WebAdmin 24Monday, May 21, 2012

HA synchronization points ๏ Transactions are uniquely identiﬁed by monotonically increasing ID ๏ All Requests from slave send the current latest txid on that slave ๏ Responses from master send back a stream of transactions that have occurred since then, along with the actual result ๏ Transaction is replayed just like when committed / recovered ๏ Nodes/Relationships touched during this application are evicted from cache to avoid cache staleness ๏ Transaction commands are only sorted when created, before stored/transmitted, thus consistency is preserved during all application phases 25Monday, May 21, 2012

Locking semantics of HA ๏ To be granted a lock the slave must have the latest version of the Node/Relationship it is trying to lock • This ensures consistency • The implementation of “Latest version ofentireNode/ Relationship” is “Latest version of the the graph” • The slave must thus sync transactions from the master 26Monday, May 21, 2012

Master election ๏ Each instance communicates/coordinates: • its latest transaction id (including the master id for that tx) • the id forclock value for when the txid was written that instance • (logical) ๏ Election chooses: 1. The instance with highest txid 2. IF multiple: The instance that was master for that tx 3. IF unavailable: The instance with the lowest clock value 4. IF multiple: The instance with the lowest id ๏ Election happens when the current master cannot be reached • Any instance can choose to re-elect • Each instance runs the election protocol individually • Notify others when election chooses new master ๏ When elected, the new master broadcasts to all instances, 27 forcing them to bind to the new masterMonday, May 21, 2012

Thank you for listening! tobias@neotechnology.com Tobias Lindaaker twitter: @thobe, #neo4j (@neo4j) web: neo4j.org neotechnology.com Hacker @ Neo Technology my web: thobe.orgMonday, May 21, 2012

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An overview of Neo4j Internals

by Tobias Lindaaker

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