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From Gilles Sadowski <>
Subject Re: [collections] BloomFilter package architecture discussion
Date Wed, 16 Oct 2019 01:08:46 GMT

2019-10-15 20:05 UTC+02:00, Claude Warren <>:
> On Tue, Oct 15, 2019 at 1:46 AM Gilles Sadowski <>
> wrote:
>> Hello.
>> > Furthermore,
>> > the other potential users and supporters have not responded to any
>> > communication about this proposal so I am floundering on that front
>> > too.
>> Who are they?
> Developers I have worked with or know of that have expertice utilizing and
> analyzing Bloom filters in various projects and papers. Apache developers
> that are considering adding Bloom filters to their projects.

For sure, it would clear the path (to a design) if more people
would lay out (usage) requirements.

>> > What I want to do is take a step back. Let’s remove the collection
>> packages
>> > from consideration and work out the base package first.
>> +1
>> > With that in mind I
>> > would like to present reasoning behind the classes as they now stand
>> > and
>> > suggest a way forward to get the core classes in before adding the
>> > collections.
>> For sure, I appreciate your willingness to provide more explanations. :-)
>> > As discussed earlier Bloom filters are a concept. In most peoples
>> > mind’s
>> > they are bit vectors with specific named functions (e.g. match => A and
>> B =
>> > A, hammingValue => cardinality(this)). This idea of the filter as a bit
>> > vector led me to using a BitSet. Originally the BloomFilter was
>> > serializable but valid arguments against that and toward making the
>> > internal representation visible enought to serialize that lead to
>> > making
>> > the BitSet public.
>> Maybe I'm missing your point, but I don't see a direct connection
>> between persistence of a "BloomFillter" instance and the visibility
>> of "BitSet" (if that is used to represent its internal state).
>> Realistically, when we “persist” a Bloom filter we are preserving the
> internal state. If the Bloom filter itself is not serializable then the
> internal state has to be preserved somehow. You made a comment on Sep 23,
> 2019 when we were discussing the serialization on the “New Sub-project
> Proposal.” topic in which you said “At first sight, I'd say that
> serialization is out-of-scope (we should let application developers deal
> with that using the available accessors).” Perhaps I have misunderstood but
> when I read that a light went on. What has to happen is that enough of the
> internal state has to be exposed for the developer to preserve that state
> and be able to reinstantiate it later. As the state is really an bit array
> I figured the most logical thing would be to expose the BitSet.
> Alternatively we could expose a ByteBuffer or any of a number of structures
> that can represent a bit array. However, the BitSet is, semantically
> speaking, what we are exposing.

Maybe I was not clear enough: I'm not saying that we should prefer
some representation (of the state) over another; only that the how
the state is represented externally need not be the same as the internal

But if the state is fully encapsulated by a "BitSet", then internal and
external can be the same, making easier both for developers and for
users.  So let's have indeed a "getBitSet()" that returns a copy of the
(private final) instance field (of type "BitSet") used to represent the
state of a "BloomFilter".

>> > In addition to the normal Bloom filter methods
>> > (hammingValue, match, etc.) there are some methods (log, cosine
>> > distance,
>> > jaccard distance, etc.) that are used by researchers and other
>> > attempting
>> > to do some work with indexing and such for large collections of
>> > filters.
>> IIUC, you indeed distinguish between core functionality (i.e. "normal
>> Bloom filter methods") and additional support for (?)...
>> If so, "core" should indeed be implemented first, with maximum
>> encapsulation (even if it would seem to prevent "research" usage).
>> Indeed I distinguish. I distinguish to align various functionality with
> external documentation. For example in Bloom’s paper he does not talk about
> Hamming values a term and concept that is found in every Bloom filter
> implementation I have seen and which is frequent in the literature. So a
> “Strict” implementation of a Bloom filter might only have a single method
> “match()” and still be considered an implementation of a Bloom filter, but
> it would not be of much use in modern systems. Realistically, there are a
> number of methods that are commonly found in Bloom filter implementations.
> I believe that at a minimum they are “match()”, “merge()” and
> “hammingValue()”.

No problem with that: API should thus contain those instance methods.
That's the kind of information we need from users of the functionality.

>> > Finally there is the question of “normal” usage. In his paper on
>> > “Compressed Bloom Filters”[1] (an implementation we do not have yet)
>> > Michael Mitzenmacher states “the Bloom filter plays a dual role. It is
>> both
>> > a data structure being used at the proxies, and a message being passed
>> > between them”. He is discussing the issue of using bloom filters in a
>> > distributed cache where the proxies distributed their bloom filter so
>> that
>> > all the proxies know which proxy might have the document. So the Bloom
>> > filter needs both an internal representation and a mechanism to pass
>> > the
>> > data between systems that use it. It seems obvious that the BitSet
>> > works
>> as
>> > the mechanism to pass the data between systems, though an immutable
>> version
>> > would be preferred.
>> So it seems we can have a Bloom filter state represented by a
>> "private" and "final" instance of "BitSet".  Is there any reason that
>> the implementation in [Collections] would ever need to use several
>> representations of its state?
>> If not, I'd think that "BloomFilter" can be a concrete class (with, for
>> now, a "BitSet" as its state).
>> The counter examples that come to mind is not within a single Bloom
>> filter
> type. A counting bloom filter tracks the number of times a bit has been set
> so that it can do deletions (something standard Bloom filters have a hard
> time with). The implementation in the code base simply extends the
> StandardBloomFilter implementation and adds a sparse array of Integers as
> the counts.

So, in my understanding, it is an extension of the above "BitSet"
state, and thus a valid case for inheriting from "BloomFilter" (as
a concrete, not "abstract", class.

> However, the counting Bloom filter does not need the BitSet
> from StandardBloomFilter. It can generate one on demand.

It can, but why would it reimplement the "getBitSet()" method,
(and more) rather than just delegate to its base class (i.e. no
duplication at all in the OO paradigm)?

> The counting Bloom
> filter needs to be able to determine if an AbstractBloomFilter matches its
> pattern. It can do this internally without using a BitSet by scanning the
> sparse array (though a bitset might prove faster). Finally the counting
> Bloom filter can be merged into another bloom filter just like any other
> AbstractBloomFilter can. It does that by basically turning on the bits in
> the AbstractBloomFilter that are indexes for the entries in the internal
> sparse array.

IIUC "match" does not need any override, while "merge" will
handle the sparse array, and then call "super.merge".  In a
"CountingBloomFilter", there will be two "merge", and it can be
construed as redundant, but I suppose that "merge" is called
much less often than "match"; hence it could be an acceptable
compromise for not having duplicate implementations (one
with a "BitSet" and one with an array of integers).
A "CountingBloomFilter" would provide an additional accessor:
"getCounts()" that returns a copy of the array of integers (i.e.
the part of the state which it manages).  [Again, it's up to the
application developer to devise the strategy for serialization.]

> There are several other examples such as layered Bloom
> filters (
> and attenuated Bloom filters (
> where
> the internal structure deviates from the BitSet model but that still need
> to present themselves as AbstractBloomFilters do. I think that Abstract
> class is the correct pattern here.

Not sure: "abstract" is adequate when several subclasses need to
override behaviours; whereas, here the behaviour is the same but
the state is expanded.

However, there may be a potential issue if the various variants above
need to be mixed and matched.  I.e. will there be
 * "LayeredBloomFilter
 * "CountingLayeredBloomFilter"
 * "AttenuatedLayeredBloomFilter"
 * "CountingAttenuatedLayeredBloomFilter"

>> > So the Bloom filter needs an external reference implementation of its
>> > state, so that it can communicate said state with other Bloom filters.
>> The
>> > quesiton of what is necessary for transmission of that state is still
>> open.
>> > I think we can agree a mechanism to identify which bits are on is
>> requried
>> > as part of the state. I have seen discussions that indicate the hashing
>> > algorithm and other configuration information should also be included,
>> but
>> > I believe this is dependent upon where the filter is being sent. All
>> Bloom
>> > filters within a system, by necessity, have to use the same hash and
>> > configuration parameters.
>> The last statement is not obvious to me, but it surely looks like
>> a simplifying assumption.
>> What are examples of "system" in that sentence?
> In my statement above a “system” is a series of processes that accept Bloom
> filters as arguments. For example a collections of 10K buckets. If you
> created Bloom filter on each bucket, gathered the buckets in to groups of
> 100 and build Bloom filters for each group you could quickly tell if and
> where an object might be by checking the group bloom filters. But every
> Bloom filter in this system must have the same “shape” (The same hash,
> number of bits, and number of functions) for the comparisons to work. All
> of those are in one “system”. However, a single application may have
> multiple systems.

So IIUC, which "hash" and how many "functions" are also part of the
state.  [Number of bits is already implicitly conveyed by "getBitSet()".]

Should the "hash" be configurable?  Or can it be an "implementation
detail" (subject to change at the discretion of the library developers)?

In the former case, I guess that we should create an "enum" that
provides all the supported algorithms.

>> > Only when sending outside the system does the
>> > other data need to be made available. In my mind this means those data
>> > structures have to be availabe to be transmitted along with the Bloom
>> > filter or filters.
>> Even if so, this is about serialization/persistence; thus, not a "core"
>> issue.
>> If the above "system" is the application level, all "BloomFilter"
>> instances
>> should be able to pass messages between them using instance methods.
> I believe this is a “core” issue. I used an example of a proxy system in my
> original post. There is no requirement that each of those proxies be on a
> single machine (logical, virtual or physical), in fact it could be argued
> that they probably would not be on the same machine. So passing Bloom
> filters between machines (I am reticent to say “system” here) is one of the
> core requirements to making Bloom filter usage efficient in modern
> distributed architectures.

If both sides of the communication channel know the exact type
of "BloomFilter" instances, the application can handle serialization
and any other required functionality that may need to persist across
the channel (e.g. the "counts").

>> > So now to the code in the current contribution. The argument that
>> > BloomFilter should be a class not an interface is a good one. However,
>> > as
>> > the internal representation of the bit structure can vary wildly,
>> Why should it?
> See response at concreate/abstract discussion above.

I need more details about the variants (cf. above indeed).

>> > I think
>> > it should be an abstract class. My reasoning is as follows: Assuming we
>> > have an external reference implementation of state many/all of the
>> methods
>> > could be implemented against that definition.
>> I don't follow: OO is about behaviour(s), "state" being an implementation
>> detail.
>> This goes to the discussion of the BloomFilterFunctions and some methods
> in the BloomFilterConfiguration class that estimate sizes. Those functions
> can be implemented against the “external reference implementation of state”
> which could be a BitSet. They all operate on the bit patterns using the
> logical “or”, “and”, “xor” functions as well as cardinality, and
> nextSetBit.

No problem, except that IMHO they are "BitSet" functions!
[And it might be worth implementing them as first class citizen,
using the functional interfaces in "java.util.function".]

>> > This is what the current
>> > implementation attempts to do, but not successfully. Any implementation
>> of
>> > the abstract class could develop an internal representation of state
>> > that
>> > is more efficient for whatever the use case is (e.g. counting Bloom
>> > filters, compressed Bloom filters, attenuated Bloom filters, etc)
>> Are these variants impossible to represent with a "BitSet"?
>> I think in all cases where the specific implementation of BloomFilter
> needs to present itself as a BloomFilter, yes BitSet can represent that.
> But is cases where the internal construct is much more complex it may not
> make sense to try to keep an internal BitSet in sync with other structures
> but rather generate the BitSet if and when it is necessary.

That's a crucial data point...
So as I wrote in the previous mail, that would indeed entail that
"BloomFilter" is an interface.  And a common base class (using
a "BitSet" for its state) makes no sense.

>> > and
>> > override the methods that are more efficiently answered within its
>> internal
>> > structure.
>> Unless I'm missing the point, this then seems to mandate an
>> interface (rather than an abstract or concrete class), contrary
>> to what was said previously...
> I think was was missing was the discussion of the Functions as noted in my
> response to your comments about OO behaviour. I am sorry that I did not
> make that clear in the original post, it probably would have reduced the
> confusion significantly. Just to be clear. I think this is an Abstract
> class that implements an “external reference implementation of state”
> interface. That interface will then allow the abstract Bloom filter class
> to implement the functions that are in the BloomFilterFunctions class (thus
> removing the utility class). In addition they will permit the
> BloomFilterConfigurtion class to continue to implement the estimated size
> methods agains the AbstractBloomFilter class. I think the implementation of
> the “external reference implementation of state” ends up being implemented
> by the concrete Bloom filter classes. I don’t like the term “external
> reference implementation of state” and wish I had a good interface name to
> apply to it, but I started with it earlier and am sticking with it here
> until it is either discarded or named.

I'm lost here; I don't know what is "the" reference state, since a
single structure cannot for example represent both a standard
filter and a counting filter...

>> > In addition, the abstract Bloom filter might implement the “data
>> > read” methods from BitSet directly. This would allow more efficient
>> > implementation of several methods that currently depend upon the
>> > creation
>> > of a BitSet and the manipulation of same.
>> Sure; if any and all variants of a "BloomFilter" implementation can
>> be created from a "BitSet", then such a factory method could be
>> part of the "Builder" interface:
>> public interface BloomFilter {
>>     // Core methods...
>>     public interface Builder {
>>         public Builder from(BitSet s); // Factory method.
>>         public BloomFilter build();
>>         // Other builder methods.
>>     }
>> }
>> Indeed, this is a valid construct. Currently the constructors take a
> ProtoBloomFilter and BloomFilterConfiguration, or a BitSet. If the
> “external reference implementation of state” is used then I expect to
> change the second constructor to take an AbstractBloomFilter instead.

Since I'm stuck with what the "reference state" is, I cannot follow...

> It is
> the ProtoBloomFilter that has the Builder. The ProtoBloomFilter allows us
> to build the hashes but not apply them to build the specific “shape”
> (definition of “shape” noted above). Thus one ProtoBloomFilter can be the
> progenitor of Bloom filters in multiple “systems” (defintion of “system”
> noted above). This also means that the 10K bucket example I noted above can
> become much more performant because we can segregate the bucket and group
> Bloom filters into different “Systems” so that we get better collision
> avoidance at the group level than we would otherwise. (I can provide data
> and example if you wish.)

Still lost; examples may help.
My impression is that we could come to that after deciding
whether single inheritance will fit the bill or not.

>> >
>> >
>> > I think this change yields the following:
>> >
>> >
>> > 1. Creation of an external reference implementation of state.
>> >
>> > 2. Conversion of BloomFilter for interface to AbstractBloomFilter class
>> >
>> > 3. Merge BloomFilterFunctions into AbstractBloomFilter
>> >
>> > 4. Rename StandardBloomFilter to BloomFilter (should we do this?)
>> >
>> > 5. Modify (Standard)BloomFilter and CountingBloomFilter to extends the
>> > AbstractBloomFilter class.
>> It seems that somehow we diverge on the emphasis given to the
>> "state".  I think that the questions which I raised above should be
>> answered before jumping to those conclusions.
> I agree and hope this response has clarified and answered.

Not totally. :-}

>> > On the topic of an external reference implementation of state, could
>> > this
>> > be an interface that AbstractBloomFilter implements? Might it comprise
>> the
>> > “read data” methods from the BitSet class?
>> IIUC, that will trivially be the case if "BitSet" can be the exchange
>> format. And how to actually perform serialization is a user's issue
>> (i.e. whether to rely on "BitSet" being "Serializable", or to implement
>> a proxy...).
>> To be honest, I had not worked through the “external reference
> implementation of state” but initially thought it might be a BitSet. After
> the above, perhaps not. Again, I haven’t quite thought it all through. If
> “external reference implementation of state” really is an interface then it
> must be possible to construct a Bloom filter from an instance of it.
> Otherwise, developers would not be able to serialize the data, deserialize
> into an instance of the interface and recreate the filter. On the other
> hand, the more complext Bloom filters might require other data (e.g. a
> sparse list of counts) to fully reconstruct them. These later cases are
> probably best considered special cases for each filter type and addressed
> within the specific implementation.

So we came to an similar conclusion that there is definitely an
issue if the "reference" is supposed to be able to represent all
the filter variants.  If this is true, then we'd have a single
"MegaBloomFilter" that combines "Standard", "Counting",
"Layered", etc. and whatever additional functionality might be
required later on...  But I may be missing many things now...

I still think that talking about serialization does not help at all.
An example (a unit test maybe) of what should be possible
in the distributed architecture which you evoked would help
fix ideas.

>> >
>> > [1]
>> >
> As an aside I was reading about "The Humpty Dumpty Principle" and came
> across
> The article makes me wish I have never used the term “external reference
> implementation of state”
> Thanks for your patience and for taking the time to read through all of
> this,
> Claude

To be pragmatic, I wonder whether I should create a branch on
within the Apache repository, so that all (including me) can
contribute and actually test the various alternatives...


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