Schema design in NoSQL is very different from schema design in a RDBMS. Once you get something like HBase up and running, you may find yourself staring blankly at a shell, lost in the possibilities of creating your first table. HBase schemas can be created or updated using the Apache HBase Shell or by using Admin in the Java API. Tables must be disabled when making ColumnFamily modifications, for example:
Configuration config = HBaseConfiguration.create();
Admin admin = new Admin(conf);
TableName table = TableName.valueOf(“myTable”);
admin.disableTable(table);
HColumnDescriptor cf1 = …;
admin.addColumn(table, cf1); // adding new ColumnFamily
HColumnDescriptor cf2 = …;
admin.modifyColumn(table, cf2);// modifying existing ColumnFamily
admin.enableTable(table);
Online schema changes are supported in the 0.92.x codebase, but the 0.90.x codebase requires the table to be disabled. When changes are made to either Tables or ColumnFamilies (e.g. region size, block size), these changes take effect the next time there is a major compaction and the StoreFiles get re-written.
Table Schema – There are many different data sets, with different access patterns and service-level expectations. Therefore, these rules of thumb are only an overview. Read the rest of this chapter to get more details after you have gone through this list.
- Aim to have regions sized between 10 and 50 GB.
- Aim to have cells no larger than 10 MB, or 50 MB if you use [mob]. Otherwise, consider storing your cell data in HDFS and store a pointer to the data in HBase.
- A typical schema has between 1 and 3 column families per table. HBase tables should not be designed to mimic RDBMS tables.
- Around 50-100 regions is a good number for a table with 1 or 2 column families. Remember that a region is a contiguous segment of a column family.
- Keep your column family names as short as possible. The column family names are stored for every value (ignoring prefix encoding). They should not be self-documenting and descriptive like in a typical RDBMS.
- If you are storing time-based machine data or logging information, and the row key is based on device ID or service ID plus time, you can end up with a pattern where older data regions never have additional writes beyond a certain age. In this type of situation, you end up with a small number of active regions and a large number of older regions which have no new writes. For these situations, you can tolerate a larger number of regions because your resource consumption is driven by the active regions only.
- If only one column family is busy with writes, only that column family accomulates memory. Be aware of write patterns when allocating resources.
Column Families – HBase currently does not do well with anything above two or three column families so keep the number of column families in your schema low. Currently, flushing and compactions are done on a per Region basis so if one column family is carrying the bulk of the data bringing on flushes, the adjacent families will also be flushed even though the amount of data they carry is small. When many column families exist the flushing and compaction interaction can make for a bunch of needless i/o. Try to make do with one column family if you can in your schemas. Only introduce a second and third column family in the case where data access is usually column scoped; i.e. you query one column family or the other but usually not both at the one time.
Where multiple ColumnFamilies exist in a single table, be aware of the cardinality (i.e., number of rows). If ColumnFamilyA has 1 million rows and ColumnFamilyB has 1 billion rows, ColumnFamilyA’s data will likely be spread across many, many regions (and RegionServers). This makes mass scans for ColumnFamilyA less efficient.
Rowkey Design
Rows in HBase are sorted lexicographically by row key. This design optimizes for scans, allowing you to store related rows, or rows that will be read together, near each other. However, poorly designed row keys are a common source of hotspotting. Hotspotting occurs when a large amount of client traffic is directed at one node, or only a few nodes, of a cluster. This traffic may represent reads, writes, or other operations. The traffic overwhelms the single machine responsible for hosting that region, causing performance degradation and potentially leading to region unavailability. This can also have adverse effects on other regions hosted by the same region server as that host is unable to service the requested load. It is important to design data access patterns such that the cluster is fully and evenly utilized.
To prevent hotspotting on writes, design your row keys such that rows that truly do need to be in the same region are, but in the bigger picture, data is being written to multiple regions across the cluster, rather than one at a time. Some common techniques for avoiding hotspotting are described below, along with some of their advantages and drawbacks.
- Salting – Salting in this sense has nothing to do with cryptography, but refers to adding random data to the start of a row key. In this case, salting refers to adding a randomly-assigned prefix to the row key to cause it to sort differently than it otherwise would. The number of possible prefixes correspond to the number of regions you want to spread the data across. Salting can be helpful if you have a few “hot” row key patterns which come up over and over amongst other more evenly-distributed rows.
- Hashing – Instead of a random assignment, you could use a one-way hash that would cause a given row to always be “salted” with the same prefix, in a way that would spread the load across the RegionServers, but allow for predictability during reads. Using a deterministic hash allows the client to reconstruct the complete rowkey and use a Get operation to retrieve that row as normal.
- Reversing the Key – A third common trick for preventing hotspotting is to reverse a fixed-width or numeric row key so that the part that changes the most often (the least significant digit) is first. This effectively randomizes row keys, but sacrifices row ordering properties.
Rows can most efficiently be read back by scanning for consecutive blocks. Say you have a table with a rowkey of customer-date-user. You can easily read back all the data for a given customer and date range using the prefix customer-first-part-of-date, but you can’t easily read back dates ranges for all users at once without scanning all the rows. If you reverse the row key and use customer-user-date, you have the reverse problem. So you want to think about what your primary read pattern is going to be when designing your keys.
HBase Compaction
Compaction, the process by which HBase cleans up after itself, comes in two flavors: major and minor. Minor compactions combine a configurable number of smaller HFiles into one larger HFile. You can tune the number of HFiles to compact and the frequency of a minor compaction. Minor compactions are important because without them, reading a particular row can require many disk reads and cause slow overall performance. A major compaction seeks to combine all HFiles into one large HFile. In addition, a major compaction does the cleanup work after a user deletes a record. When a user issues a Delete call, the HBase system places a marker in the key-value pair so that it can be permanently removed during the next major compaction.
Minor Compaction – HBase will automatically pick some smaller HFiles and rewrite them into fewer bigger Hfiles. This process is called minor compaction. Minor compaction reduces the number of storage files by rewriting smaller files into fewer but larger ones, performing a merge sort.
Major Compaction – Major compaction merges and rewrites all the HFiles in a region to one HFile per column family, and in the process, drops deleted or expired cells. This improves read performance; however, since major compaction rewrites all of the files, lots of disk I/O and network traffic might occur during the process. This is called write amplification.
Major compactions can be scheduled to run automatically. Due to write amplification, major compactions are usually scheduled for weekends or evenings. Note that MapR-DB has made improvements and does not need to do compactions. A major compaction also makes any data files that were remote, due to server failure or load balancing, local to the region server.
Region Split
Initially there is one region per table. When a region grows too large, it splits into two child regions. Both child regions, representing one-half of the original region, are opened in parallel on the same Region server, and then the split is reported to the HMaster. For load balancing reasons, the HMaster may schedule for new regions to be moved off to other servers.
Read Load Balancing
Splitting happens initially on the same region server, but for load balancing reasons, the HMaster may schedule for new regions to be moved off to other servers. This results in the new Region server serving data from a remote HDFS node until a major compaction moves the data files to the Regions server’s local node. HBase data is local when it is written, but when a region is moved (for load balancing or recovery), it is not local until major compaction.
HDFS Data Replication
All writes and Reads are to/from the primary node. HDFS replicates the WAL and HFile blocks. HFile block replication happens automatically. HBase relies on HDFS to provide the data safety as it stores its files. When data is written in HDFS, one copy is written locally, and then it is replicated to a secondary node, and a third copy is written to a tertiary node.
The WAL file and the Hfiles are persisted on disk and replicated, so how does HBase recover the MemStore updates not persisted to HFiles?
HBase Crash Recovery
When a RegionServer fails, Crashed Regions are unavailable until detection and recovery steps have happened. Zookeeper will determine Node failure when it loses region server heart beats. The HMaster will then be notified that the Region Server has failed.
When the HMaster detects that a region server has crashed, the HMaster reassigns the regions from the crashed server to active Region servers. In order to recover the crashed region server’s memstore edits that were not flushed to disk. The HMaster splits the WAL belonging to the crashed region server into separate files and stores these file in the new region servers’ data nodes. Each Region Server then replays the WAL from the respective split WAL, to rebuild the memstore for that region.
Data Recovery
WAL files contain a list of edits, with one edit representing a single put or delete. Edits are written chronologically, so, for persistence, additions are appended to the end of the WAL file that is stored on disk.
What happens if there is a failure when the data is still in memory and not persisted to an HFile? The WAL is replayed. Replaying a WAL is done by reading the WAL, adding and sorting the contained edits to the current MemStore. At the end, the MemStore is flush to write changes to an HFile.
