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Persistent references

ATLAS software meeting Database session. Persistent references. David Adams BNL May 28, 2002. Contents. Comment Definitions Use cases Structure ID’s Navigation. Comment. This is a first draft of a talk for the LGC workshop next week

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Persistent references

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  1. ATLAS software meeting Database session Persistent references David Adams BNL May 28, 2002

  2. Contents • Comment • Definitions • Use cases • Structure • ID’s • Navigation Persistent refs ATLAS SW week – DB session

  3. Comment • This is a first draft of a talk for the LGC workshop next week • It merges the BNL HES model with the LCG persistency RTAG document Persistent refs ATLAS SW week – DB session

  4. Definitions • DO (data object) • Unit of persistent data • HOS (hybrid object store) • Persistent store for DO’s • EDO (event data object) • Unit of event data (i.e. data associated with a triggered beam crossing) • HES (hybrid event store) • Persistent store for EDO’s Persistent refs ATLAS SW week – DB session

  5. Definitions (cont) • OID – object identifier • Unique identifier for a DO • EDOID – EDO identifier • Unique identifier for an EDO • Special case of OID Persistent refs ATLAS SW week – DB session

  6. Definitions (cont) • Event ID • Unique identifier for a triggered beam crossing • E.g. a 32-bit int • Content ID • Identifier which labels the content of an EDO • In StoreGate this is the EDO transient type and a string key Persistent refs ATLAS SW week – DB session

  7. Use cases • HOS access • Insert object into store and receive OID • Present OID to store and obtain access to EDO • HES access • Insert object into store and receive OID • Present OID to store and obtain access to EDO • Restricted view • Define a restricted view of store • Query view for EDO (or any DO) Persistent refs ATLAS SW week – DB session

  8. Use cases (cont) • Replication • File replication • Object replication (between files) • From one storage technology to another • E.g. optimizing for space and then optimizing for fast access Persistent refs ATLAS SW week – DB session

  9. Use cases (cont) • References • One transient DO holds a reference to another • Both are written to the store • Both are read back into a transient store and the reference is restored • Location • In the previous only the first object is read in • There is a mechanism to locate the second object using the “reference” from the first Persistent refs ATLAS SW week – DB session

  10. Use cases (cont) • Replicated object references • The objects in the references use case are replicated • Either the original or replica of each object is read in • The reference is restore for any pair of objects Persistent refs ATLAS SW week – DB session

  11. Use cases (cont) • Location of replicated object • In the previous example either the original or replica of the first object is read in • The mechanism for locating the referenced object may find either the original or the replica of the second object Persistent refs ATLAS SW week – DB session

  12. Structure • Files • Objects reside in files • Files may be implemented in different PT’s (persistence technologies) • Files have a common HOS interface • HES • Layer over HES which takes advantage of the independence of data from different events Persistent refs ATLAS SW week – DB session

  13. Structure (cont) • HOS navigation • Simple means to use a DOID locate a DO in a collection of files • HES navigation • Same for EDOID and EDO • High level navigation • Other mechanisms for locating objects or files • Multiple implementations • Many use relational DB Persistent refs ATLAS SW week – DB session

  14. Structure (cont) Persistent refs ATLAS SW week – DB session

  15. Structure (cont) • HOS file interface • File(OID)  DO • HES file interface • File(EDOID)  EDO • File()  {Event ID} • File()  {Content ID} • File(Event ID)  {Content ID} • File(Event ID, Content ID)  EDO Persistent refs ATLAS SW week – DB session

  16. Identifiers • Options for generation • Standalone  128 bit ID • Central source  64+ bits • Hand out blocks of ID’s to reduce network access • To site, file system, compute node Persistent refs ATLAS SW week – DB session

  17. Identifiers (cont) • Content of HOS identifier • Simplest and smallest if ID has no content (other than uniqueness) • Including File ID provides aid to navigation and path to uniqueness • Should we also add a type ID? • Content of HES identifier • Could add event ID • Could add content ID • Type and string key in ATLAS Persistent refs ATLAS SW week – DB session

  18. Navigation • Low level HOS navigation • Search a collection of files until an OID is found • Might do some simple cataloging to optimize searches and cache results Persistent refs ATLAS SW week – DB session

  19. Navigation (cont) • High level navigation • Implemented above HES/HOS • Is able to restrict data to certain types, e.g. geometry, calibration or event data • Could provide catalogs based on history of data generation • Could identify small (and thus easily searchable) collections of relevant data • Datasets Persistent refs ATLAS SW week – DB session

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