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Machi cluster-of-clusters "name game" sketch

-- mode: org; --

1. "Name Games" with random-slicing style consistent hashing

Our goal: to distribute lots of files very evenly across a cluster of Machi clusters (hereafter called a "cluster of clusters" or "CoC").

2. Assumptions

Basic familiarity with Machi high level design and Machi's "projection"

The Machi high level design document contains all of the basic background assumed by the rest of this document.

Analogy: "neighborhood : city :: Machi : cluster-of-clusters"

Analogy: The word "machi" in Japanese means small town or neighborhood. As the Tokyo Metropolitan Area is built from many machis and smaller cities, therefore a big, partitioned file store can be built out of many small Machi clusters.

Familiarity with the Machi cluster-of-clusters/CoC concept

It's clear (I hope!) from the Machi high level design document that Machi alone does not support any kind of file partitioning/distribution/sharding across multiple small Machi clusters. There must be another layer above a Machi cluster to provide such partitioning services.

The name "cluster of clusters" originated within Basho to avoid conflicting use of the word "cluster". A Machi cluster is usually synonymous with a single Chain Replication chain and a single set of machines (e.g. 2-5 machines). However, in the not-so-far future, we expect much more complicated patterns of Chain Replication to be used in real-world deployments.

"Cluster of clusters" is clunky and long, but we haven't found a good substitute yet. If you have a good suggestion, please contact us! ^_^

Using the cluster-of-clusters quick-and-dirty prototype as an architecture sketch, let's now assume that we have n independent Machi clusters. We assume that each of these clusters has roughly the same chain length in the nominal case, e.g. chain length of 3. We wish to provide partitioned/distributed file storage across all n clusters. We call the entire collection of n Machi clusters a "cluster of clusters", or abbreviated "CoC".

We may wish to have several types of Machi clusters, e.g. chain length of 3 for normal data, longer for cannot-afford-data-loss files, and shorter for don't-care-if-it-gets-lost files. Each of these types of chains will have a name N in the CoC namespace. The role of the CoC namespace will be demonstrated in Section 3 below.

Continue CoC prototype's assumption: a Machi cluster is unaware of CoC

Let's continue with an assumption that an individual Machi cluster inside of the cluster-of-clusters is completely unaware of the cluster-of-clusters layer.

TODO: We may need to break this assumption sometime in the future?

The reader is familiar with the random slicing technique

I'd done something very-very-nearly-identical for the Hibari database 6 years ago. But the Hibari technique was based on stuff I did at Sendmail, Inc, so it felt old news to me. {shrug}

The Hibari documentation has a brief photo illustration of how random slicing works, see Hibari Sysadmin Guide, chain migration

For a comprehensive description, please see these two papers:

Reliable and Randomized Data Distribution Strategies for Large Scale Storage Systems Alberto Miranda et al. http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.226.5609 (short version, HIPC'11)

Random Slicing: Efficient and Scalable Data Placement for Large-Scale Storage Systems Alberto Miranda et al. DOI: http://dx.doi.org/10.1145/2632230 (long version, ACM Transactions on Storage, Vol. 10, No. 3, Article 9, 2014)

CoC locator: We borrow from random slicing but do not hash any strings!

We will use the general technique of random slicing, but we adapt the technique to fit our use case.

In general, random slicing says:

  • Hash a string onto the unit interval [0.0, 1.0)
  • Calculate h(unit interval point, Map) -> bin, where Map partitions the unit interval into bins.

Our adaptation is in step 1: we do not hash any strings. Instead, we store & use the unit interval point as-is, without using a hash function in this step. This number is called the "CoC locator".

As described later in this doc, Machi file names are structured into several components. One component of the file name contains the "CoC locator"; we use the number as-is for step 2 above.

3. A simple illustration

We use a variation of the Random Slicing hash that we will call rs_hash_with_float(). The Erlang-style function type is shown below.

%% type specs, Erlang-style
-spec rs_hash_with_float(float(), rs_hash:map()) -> rs_hash:cluster_id().

I'm borrowing an illustration from the HibariDB documentation here, but it fits my purposes quite well. (I am the original creator of that image, and also the use license is compatible.)

/greg/machi/media/commit/30000d6602a350fdc3e1ff12e0450a63ee68c76f/doc/cluster-of-clusters/migration-4.png

Illustration of 'Map', using four Machi clusters

Assume that we have a random slicing map called Map. This particular Map maps the unit interval onto 4 Machi clusters:

Hash range Cluster ID
0.00 - 0.25 Cluster1
0.25 - 0.33 Cluster4
0.33 - 0.58 Cluster2
0.58 - 0.66 Cluster4
0.66 - 0.91 Cluster3
0.91 - 1.00 Cluster4

Assume that the system chooses a CoC locator of 0.05. According to Map, the value of rs_hash_with_float(0.05,Map) = Cluster1. Similarly, rs_hash_with_float(0.26,Map) = Cluster4.

4. An additional assumption: clients will want some control over file location

We will continue to use the 4-cluster diagram from the previous section.

Our new assumption: client control over initial file location

The CoC management scheme may decide that files need to migrate to other clusters. The reason could be for storage load or I/O load balancing reasons. It could be because a cluster is being decommissioned by its owners. There are many legitimate reasons why a file that is initially created on cluster ID X has been moved to cluster ID Y.

However, there are also legitimate reasons for why the client would want control over the choice of Machi cluster when the data is first written. The single biggest reason is load balancing. Assuming that the client (or the CoC management layer acting on behalf of the CoC client) knows the current utilization across the participating Machi clusters, then it may be very helpful to send new append() requests to under-utilized clusters.

5. Use of the CoC namespace: name separation plus chain type

Let us assume that the CoC framework provides several different types of chains:

Chain length CoC namespace Mode Comment
3 normal AP Normal storage redundancy & cost
2 cheap AP Reduced cost storage
1 risky AP Really cheap storage
9 paranoid AP Safety-critical storage
3 sequential CP Strong consistency

The client may want to choose the amount of redundancy that its application requires: normal, reduced cost, or perhaps even a single copy. The CoC namespace is used by the client to signal this intention.

Further, the CoC administrators may wish to use the namespace to provide separate storage for different applications. Jane's application may use the namespace "jane-normal" and Bob's app uses "bob-cheap". The CoC administrators may definite separate groups of chains on separate servers to serve these two applications.

6. Floating point is not required … it is merely convenient for explanation

NOTE: Use of floating point terms is not required. For example, integer arithmetic could be used, if using a sufficiently large interval to create an even & smooth distribution of hashes across the expected maximum number of clusters.

For example, if the maximum CoC cluster size would be 4,000 individual Machi clusters, then a minimum of 12 bits of integer space is required to assign one integer per Machi cluster. However, for load balancing purposes, a finer grain of (for example) 100 integers per Machi cluster would permit file migration to move increments of approximately 1% of single Machi cluster's storage capacity. A minimum of 12+7=19 bits of hash space would be necessary to accommodate these constraints.

It is likely that Machi's final implementation will choose a 24 bit integer to represent the CoC locator.

7. Proposal: Break the opacity of Machi file names

Machi assigns file names based on:

ClientSuppliedPrefix ++ "^" ++ SomeOpaqueFileNameSuffix

What if the CoC client could peek inside of the opaque file name suffix in order to remove (or add) the CoC location information that we need?

The notation we use

  • T = the target CoC member/Cluster ID chosen by the CoC client at the time of append()
  • p = file prefix, chosen by the CoC client.
  • L = the CoC locator
  • N = the CoC namespace
  • u = the Machi file server unique opaque file name suffix, e.g. a GUID string
  • F = a Machi file name, i.e., p^L^N^u

The details: CoC file write

  1. CoC client chooses p, T, and N (i.e., the file prefix, target cluster, and target cluster namespace)
  2. CoC client knows the CoC Map for namespace N.
  3. CoC client choose some CoC locator value L such that rs_hash_with_float(L,Map) = T (see below).
  4. CoC client sends its request to cluster T: append_chunk(p,L,N,...) -> {ok,p^L^N^u,ByteOffset}
  5. CoC stores/uses the file name F = p^L^N^u.

The details: CoC file read

  1. CoC client knows the file name F and parses it to find the values of L and N (recall, F = p^L^N^u).
  2. CoC client knows the CoC Map for type N.
  3. CoC calculates rs_hash_with_float(L,Map) = T
  4. CoC client sends request to cluster T: read_chunk(F,...) -> … success!

The details: calculating 'L' (the CoC locator) to match a desired target cluster

  1. We know Map, the current CoC mapping for a CoC namespace N.
  2. We look inside of Map, and we find all of the unit interval ranges that map to our desired target cluster T. Let's call this list MapList = [Range1=(start,end],Range2=(start,end],...].
  3. In our example, T=Cluster2. The example Map contains a single unit interval range for Cluster2, [(0.33,0.58]].
  4. Choose a uniformly random number r on the unit interval.
  5. Calculate locator L by mapping r onto the concatenation of the CoC hash space range intervals in MapList. For example, if r=0.5, then L = 0.33 + 0.5*(0.58-0.33) = 0.455, which is exactly in the middle of the (0.33,0.58] interval.

8. File migration (a.k.a. rebalancing/reparitioning/resharding/redistribution)

What is "migration"?

This section describes Machi's file migration. Other storage systems call this process as "rebalancing", "repartitioning", "resharding" or "redistribution". For Riak Core applications, it is called "handoff" and "ring resizing" (depending on the context). See also the Hadoop file balancer for another example of a data migration process.

As discussed in section 5, the client can have good reason for wanting to have some control of the initial location of the file within the cluster. However, the cluster manager has an ongoing interest in balancing resources throughout the lifetime of the file. Disks will get full, hardware will change, read workload will fluctuate, etc etc.

This document uses the word "migration" to describe moving data from one Machi chain to another within a CoC system.

A simple variation of the Random Slicing hash algorithm can easily accommodate Machi's need to migrate files without interfering with availability. Machi's migration task is much simpler due to the immutable nature of Machi file data.

Change to Random Slicing

The map used by the Random Slicing hash algorithm needs a few simple changes to make file migration straightforward.

  • Add a "generation number", a strictly increasing number (similar to a Machi cluster's "epoch number") that reflects the history of changes made to the Random Slicing map
  • Use a list of Random Slicing maps instead of a single map, one map per chance that files may not have been migrated yet out of that map.

As an example:

/greg/machi/media/commit/30000d6602a350fdc3e1ff12e0450a63ee68c76f/doc/cluster-of-clusters/migration-3to4.png

Illustration of 'Map', using four Machi clusters

And the new Random Slicing map for some CoC namespace N might look like this:

Generation number / Namespace 7 / cheap
SubMap 1
Hash range Cluster ID
0.00 - 0.33 Cluster1
0.33 - 0.66 Cluster2
0.66 - 1.00 Cluster3
SubMap 2
Hash range Cluster ID
0.00 - 0.25 Cluster1
0.25 - 0.33 Cluster4
0.33 - 0.58 Cluster2
0.58 - 0.66 Cluster4
0.66 - 0.91 Cluster3
0.91 - 1.00 Cluster4

When a new Random Slicing map contains a single submap, then its use is identical to the original Random Slicing algorithm. If the map contains multiple submaps, then the access rules change a bit:

  • Write operations always go to the newest/largest submap.

  • Read operations attempt to read from all unique submaps.

    • Skip searching submaps that refer to the same cluster ID.

      • In this example, unit interval value 0.10 is mapped to Cluster1 by both submaps.
    • Read from newest/largest submap to oldest/smallest submap.
    • If not found in any submap, search a second time (to handle races with file copying between submaps).
    • If the requested data is found, optionally copy it directly to the newest submap. (This is a variation of read repair (RR). RR here accelerates the migration process and can reduce the number of operations required to query servers in multiple submaps).

The cluster-of-clusters manager is responsible for:

  • Managing the various generations of the CoC Random Slicing maps for all namespaces.
  • Distributing namespace maps to CoC clients.
  • Managing the processes that are responsible for copying "cold" data, i.e., files data that is not regularly accessed, to its new submap location.
  • When migration of a file to its new cluster is confirmed successful, delete it from the old cluster.

In example map #7, the CoC manager will copy files with unit interval assignments in (0.25,0.33], (0.58,0.66], and (0.91,1.00] from their old locations in cluster IDs Cluster1/2/3 to their new cluster, Cluster4. When the CoC manager is satisfied that all such files have been copied to Cluster4, then the CoC manager can create and distribute a new map, such as:

Generation number / Namespace 8 / cheap
SubMap 1
Hash range Cluster ID
0.00 - 0.25 Cluster1
0.25 - 0.33 Cluster4
0.33 - 0.58 Cluster2
0.58 - 0.66 Cluster4
0.66 - 0.91 Cluster3
0.91 - 1.00 Cluster4

The HibariDB system performs data migrations in almost exactly this manner. However, one important limitation of HibariDB is not being able to perform more than one migration at a time. HibariDB's data is mutable, and mutation causes many problems already when migrating data across two submaps; three or more submaps was too complex to implement quickly.

Fortunately for Machi, its file data is immutable and therefore can easily manage many migrations in parallel, i.e., its submap list may be several maps long, each one for an in-progress file migration.

9. Other considerations for FLU/sequencer implementations

Append to existing file when possible

In the earliest Machi FLU implementation, it was impossible to append to the same file after ~30 seconds. For example:

  • Client: append(prefix="foo",...) -> {ok,"foo^suffix1",Offset1}
  • Client: append(prefix="foo",...) -> {ok,"foo^suffix1",Offset2}
  • Client: append(prefix="foo",...) -> {ok,"foo^suffix1",Offset3}
  • Client: sleep 40 seconds
  • Server: after 30 seconds idle time, stop Erlang server process for the "foo^suffix1" file
  • Client: …wakes up…
  • Client: append(prefix="foo",...) -> {ok,"foo^suffix2",Offset4}

Our ideal append behavior is to always append to the same file. Why? It would be nice if Machi didn't create zillions of tiny files if the client appends to some prefix very infrequently. In general, it is better to create fewer & bigger files by re-using a Machi file name when possible.

The sequencer should always assign new offsets to the latest/newest file for any prefix, as long as all prerequisites are also true,

  • The epoch has not changed. (In AP mode, epoch change -> mandatory file name suffix change.)
  • The latest file for prefix p is smaller than maximum file size for a FLU's configuration.

10. Acknowledgments

The source for the "migration-4.png" and "migration-3to4.png" images come from the HibariDB documentation.