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From "Barona, Ricardo" <>
Subject Re: Spark EM LDA Optimizer support
Date Fri, 21 Jul 2017 18:44:23 GMT
Once a saved model is loaded it needs to be converted to LocalLDAModel if it’s a DistributedLDAModel
but from what I heard, the importance of what you used for training, EM and Online is in the
topics matrix that generates one and the other. I’m not exactly and expert but I’d think
they are going to be different, right? The topics matrix of a LocalLDAModel coming from DistributedLDAModel
will remain the same and topic distributions will be calculated based on that. 

On 7/21/17, 1:26 PM, "Edwards, Brandon" <> wrote:

    A question just came up for me. Is there a true use case for utilizing EM that allows
one to carry context from previous models into the future? It seems that once you save to
a local model in order to utilize it for future data, from then on you only can use the Online
optimizer. If this is correct, I vote for getting rid of EM. I don’t see value in supporting
a use case that does not carry context into future models.
    On 7/21/17, 11:08 AM, "Barona, Ricardo" <> wrote:
        During the last 9 days, I've been working on modifying Apache Spot LDA wrapper to
enable the possibility of saving models and load existing models and then get topic distributions
for the same corpus or for new documents (see
Until now, Apache Spot ML module has been running in batch mode training and getting topic
distributions with the same documents it trained but that needs to change soon as we are looking
forward to achieving near real time.
        Since this year, Apache Spot enabled Online optimizer so users can select whether
to run LDA using EM or Online; EM was the first option we implemented and then we decided
it was a good idea to offer Online as well.
        In my intention for keep supporting both, EM and Online optimizer, I modified the
code in such way that you can train with either one but only get topic distributions with
LocalLDAModel. The reason for that is that only LocalLDAModel supports getting topic distributions
for new documents. The problem with that approach is that a very simple unit test we have
is failing now and the it is because when I convert DistributedLDAModel to LocalLDAModel,
the document concentration parameter remains the same as it was originally provided for EM
but it doesn't necessarily work for LocalLDAModel.topicDistributions method.
        Take a look at
There you can see the expected result from training and getting topic distributions with EM
only or Online only in a two document one word each document data set.
        Then, here is the problem I explained before about converting DistributedLDAModel
to LocalLDAModel:
        A possible solution for this is to use the following code to implement a custom function
to convert DistributedLDAModel to LocalLDAModel (see
and the code below):
        package org.apache.spark.mllib.clustering
        import org.apache.spark.mllib.linalg.{Matrix, Vector}
        object SpotLDA {
            * Creates a new LocalLDAModel but it can reset alpha and beta (although we just
need alpha).
            * @param topicsMatrix Distributed LDA Model topicsMatrix
            * @param alpha New value for alpha i.e. If Model was trained with 1.002 for alpha
using EM optimizer, this method
            *              allows you to reset alpha to something like 0.0009 and get topic
distributions with the desired
            *              document concentration.
            * @param beta New value for beta
            * @return LocalLDAModel
          def toLocal(topicsMatrix: Matrix, alpha: Vector, beta: Double): LocalLDAModel ={
            new LocalLDAModel(topicsMatrix, alpha, beta)
        The only disadvantage I see here is that users will need to provide 3 parameters if
they are using EM optimizer instead of only 2:
        -          EM alpha
        -          EM beta
        -          Online alpha
        Or provide only 2 parameters if they prefer to work with Online Optimizer only
        -          Online alpha
        -          Online beta
        Discussing this with Gustavo, he suggested we even set a “default” number for
Online alpha so if users only configure EM alpha and EM beta the application will keep working.
        Being said all that, here is the big question I’d like to ask: should we keep supporting
both, EM Optimizer and Online Optimizer and have users to configure the required parameters
or do you think is time to let EM go and just keep Online optimizer?
        My vote is for keep both but let me know if what you think.
        Ricardo Barona

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