HBase Prototype
Indroduction
ApacheDS Partition Design
This page describes a partition which stores data in Hadoop's HBase database.
Introduction
The HBase partition is implemented as XDBM Partition. The main advantage to an implementation of the simple Partition interface is that the powerful search engine can be used. However instead of storing entries by their DN an hierarchical approach is used, inspired by http://cwiki.apache.org/confluence/display/DIRxSRVx11/Xdbm+fast+modifyDn+proposal and http://www.openldap.org/conf/odd-sfo-2003/howard-dev.pdf.
In contrast to the JDBM (or the LDIF) partition, the directory server (ApacheDS) and the storage enging (HBase) don't run within the same JVM but are distributed over the network and communicate via RPC calls. Hence it is important to reduce the communication between ApacheDS and HBase to a minimum.
HBase Schema Design
The following format is used to illustrate the HBase table layout, timestamps are omitted:
| Code Block |
|---|
------------------------------------------------------------
| table name | family1 | family2 |
------------------------------------------------------------
| row1 | qualifier1 -> value1 | qualifierA -> valueA |
| | qualifier2 -> value2 | qualifierB -> valueB |
------------------------------------------------------------
| row2 | qualifier1 -> value1 | qualifierA -> valueA |
| | qualifier2 -> value2 | qualifierB -> valueB |
------------------------------------------------------------
|
Master Table
The master table stores the entries, one entry per row:
| Code Block |
|---|
--------------------------------------------------------------------------------------------------
| master | treeInfo | upAttributes |
--------------------------------------------------------------------------------------------------
| 0 | sequence -> 8216 | |
--------------------------------------------------------------------------------------------------
| 1 | parentId -> 0 | objectClass0 -> top |
| | upRdn -> o=sevenSeas | objectClass1 -> organization |
| | normRdn -> 2.5.4.10=sevenseas | o0 -> sevenSeas |
--------------------------------------------------------------------------------------------------
| 2 | parentId -> 1 | objectClass0 -> top |
| | upRdn -> ou=people | objectClass1 -> organizationalUnit |
| | normRdn -> 2.5.4.11=people | ou0 -> people |
--------------------------------------------------------------------------------------------------
| 3 | parentId -> 1 | objectClass0 -> top |
| | upRdn -> ou=groups | objectClass1 -> organizationalUnit |
| | normRdn -> 2.5.4.11=groups | ou0 -> groups |
--------------------------------------------------------------------------------------------------
| 6 | parentId -> 2 | objectClass0 -> top |
| | upRdn -> cn=Horatio Hornblower | objectClass1 -> person |
| | normRdn -> 2.5.4.3=horatio hornblower | objectClass2 -> organizationalPerson |
| | | objectClass3 -> inetOrgPerson |
| | | cn0 -> Horatio Hornblower |
| | | description0 -> Capt. Horatio Hornblower, R.N |
| | | givenName0 -> Horatio |
| | | sn0 -> Hornblower |
| | | uid0 -> hhornblo |
| | | mail0 -> hhornblo@royalnavy.mod.uk |
| | | userPassword0 -> <bytes> |
--------------------------------------------------------------------------------------------------
| ... | | |
--------------------------------------------------------------------------------------------------
|
The row key is a sequentially generated 8-byte long. An advantage of using longs is compatibility with XDBM. A disadvantage is that sequential keys are not optimal for distributing data and load balancing across data nodes.
The 'treeInfo' column family stores hierarchical information:
- Column 'parentId' contains the row key of the parent entry.
- Column 'upRdn' contains the user provided local name, relative to the parent (normally the RDN; the suffix DN for the context entry). It is used to reconstruct the entry's DN.
- Column 'normRdn' contains the normalized local name, relative to the parent (normally the RDN; DN for the context entry). This is just an optimization for constructing the key of the tree table in order to avoid RDN normalization (see below).
The 'upAttributes' column family contains a map with all the attributes.
- The attribute description (type+options) is used as column qualifier.
- HBase stores one value per column. There are several workarounds how to store multiple values. When using serialization or JSON format it won't be possible to access one value at a time. Hence each value is stored in its own column and an additional index is added to the column qualifier. This way each value can be read and written separately.
- The additional index is a zero-based 4-byte signed integer. To reconstruct the user provided attribute description the last 4 bytes needs to be removed from the column qualifier.
- The values are stored as byte[].
The row with key '0' is a special row, it's the virtual root. Its column 'treeInfo:sequence' contains the row key sequence number. HBase provides an atomic increment-and-get operation to obtain the next key for a new entry.
To retrieve the DN of an entry by its ID the entry's RDN and parent ID must be fetched. As long as the parent ID is greather than 0 this step must be repeated for all parents. The DN is the result of all concatenated RDNs.
It would be also possible to determine the ID for an DN, however a full table scan would be necessary. For this reason a second table is available.
The master table contains all information needed to restore the data: reference to the parent, user provided RDN, and user provided attributes.
Alternatives and Improvements:
- It would be possible to store the serialized form of the server entry instead of the attributes.
- It would be possible to store the serialized form of the RDN to avoid parsing.
- Different kind of attributes could be stored in separate column families (e.g. binary attributes).
- Usage of entry UUID as row key. This helps to distribute the entries over all clusters and may avoid hot spots.
- Add oneLevelCount and subLevelCount (currently stored in tree table) for faster lookup of counts.
- Add normAttributes (currently stored in tree table) for faster lookup of reverse index and evaluator.
...