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From Matt Foley <ma...@apache.org>
Subject Re: [DISCUSS] Using JSON Path to support more complex documents with the JSONMap Parser
Date Thu, 25 Jan 2018 20:06:26 GMT
Heh, as I said, I was looking in Python.  For SAX-like JSON parsers I found numerous libraries,
most built on top of an underlying Python library named ijson, which is itself based on a
C library called yajl.

The yajl page (http://lloyd.github.io/yajl/ ) lists a double handful of language bindings
but, annoyingly, none for Java; nor does Google seem to know of any.

In Java, there's a library named json-simple in the Google Code Archive which claims a SAX-like
interface and broad production-level adoption/robustness: https://code.google.com/archive/p/json-simple/
.  I don't have experience with it.

Of course, the gold standard json library for Java is Jackson.  It documents stream-based
parsing, but not "SAX-like".
https://github.com/FasterXML/jackson-docs/wiki/JacksonStreamingApi indicates that using it
is equivalent to writing a parser, which suggests (disappointingly) somewhat lower-level than
SAX api.
http://www.cowtowncoder.com/blog/archives/2009/01/entry_132.html compares Jackson streaming
interface to Stax and SAX, and says it is like Stax Cursor api, claiming simpler use than
SAX (about which I have no opinion).
So I think most people use Jackson for non-streaming consumption of json.

JsonPath implementation uses Jackson under the hood, which seems good to me -- professionals
don't recreate the wheel.
And it has the charm (for this community) of a DSL-like interface.  It's likely a good choice.

Hope this helps,
--Matt

On 1/25/18, 10:05 AM, "Otto Fowler" <ottobackwards@gmail.com> wrote:

    In other words, I don’t believe the issue is parsing, but rather searching
    and extracting.
    
    I have used SAX with xml as well, can you point me to the json equivalent
    you found?
    
    
    On January 25, 2018 at 13:01:58, Otto Fowler (ottobackwards@gmail.com)
    wrote:
    
    JSONPath is indeed what nifi uses.  I used their implementation as a guide.
    I believe starting with a path would be a good minimum viable, a good start.
    We could support multiple paths of course.
    
    Beside the fact that I knew NiFi used this approach, I believe that
    JSONPath provides a flexible mechanism for defining
    the targets within the document, and would make this more usable across
    various document structures.
    
    We already do full document with simple json btw.
    
    On January 25, 2018 at 12:45:12, Matt Foley (mattf@apache.org) wrote:
    
    Hi Otto,
    Oddly, I had reason a couple weeks ago to try to figure out a streaming
    parser for very large json objects -- altho it was in Python rather than
    Java.
    Search showed two basic approaches, both unsurprisingly modeled on xml
    processing:
    - SAX-like parsing
    - XPath-like parsing
    
    Both are capable of true streaming interface, that is one doesn't have to
    load the whole json into memory first.
    The sound-bite comparison of the two, thanks to stackoverflow, is:
    
    > SAX is a top-down parser and allows serial access to a XML document, and
    works well for read only [serial, streamed] access.
    > XPath is useful when you only need a couple of values from the XML
    document, and you know where to find them (you know the path of the data,
    /root/item/challange/text).
    > [XPath is] certainly easier to use, ... whereas ... SAX will always be a
    lot more awkward to program than XPath.
    
    Having used SAX before, I agree it's got an "awkward" api, but it's quite
    usable and does the job.
    I haven't been hands-on with XPath.
    
    Is XPath (or rather JSONPath) what NiFi uses?
    And is it sufficient for our needs to have a fixed path to the message
    sequence in any given json bundle?
    
    Thanks,
    --Matt
    
    
    On 1/25/18, 7:57 AM, "Otto Fowler" <ottobackwards@gmail.com> wrote:
    
    While it would be preferred if all data streamed into the parsers is
    already in ‘stream’ form, as opposed to ‘batched’ form, it may not always
    be possible, or possible at every step of system development.
    
    I was wondering if it would be worth adding optional support to the JSONMap
    Parser to support more complex documents, and split them in the parser into
    multiple messages. This is similar in function to the JSON Splitter
    processor in NiFi
    
    So, a document would come into the JSONMap Parser from Kafka, with some
    embedded set of the real message content, such as in this simplified
    example:
    
    {
    “messages" : [
    { message1},
    { message2},
    ….
    {messageN}
    ]
    }
    
    the JSONMap Parser, would have a new configuration item for message
    selection, that would be a JSON Path expression
    
    “messageSelector” : “$.messages “
    
    Inside the JSONMap Parser, it would evaluate the expression, and do the
    same processing on each item returned by the expression list.
    
    the Parser interface already supports returning multiple message objects
    from a single byte[] input.
    
    There is a performance penalty to be paid here, and it is more than just
    doing more than one message due to the JSONPath evaluation.
    
    I can see this being useful in a couple of circumstances:
    
    -
    
    You want to work with some document format with metron but do not have
    NiFi or the equivalent available or setup yet
    -
    
    You want to prototype with Metron before you get the ‘preprocessing’
    setup
    -
    
    You are not going to be able to use NiFi and are ok with the performance
    
    I have something in github to look at for more detail :
    ottobackwards/json-path-play
    <https://github.com/ottobackwards/json-path-play>
    
    Thoughts?
    



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