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https://issues.apache.org/jira/browse/AVRO-1467?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13915903#comment-13915903
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Jim Pivarski commented on AVRO-1467:
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Hi! Working through your bullet points here...
Thanks for pointing me to
org.apache.avro.SchemaCompatibility.checkReaderWriterCompatibility (from
AVRO-1409): it's exactly what I was looking for. Something like that ought to
be briefly mentioned on the Getting Started (Java) page.
My examples work in this per-schema validation, including names and including
aliases, but my bug report about the per-instance validation stands.
Does decoding always go through the following sequence: (1) read embedded
schema, (2) loop through data instances? If so, then the problem of a
too-permissive instance validation can be solved by doing a one-time schema
validation at the beginning of reading, using checkReaderWriterCompatibility,
which does the full tests. In fact, that may be preferable to doing redundant
checks on every instance.
I'm going to look into aliases in more detail, and if I find what looks like a
bug, I'll file another bug report. I won't bother checking the Python version
because of its lack of support.
> Schema resolution does not check record names
> ---------------------------------------------
>
> Key: AVRO-1467
> URL: https://issues.apache.org/jira/browse/AVRO-1467
> Project: Avro
> Issue Type: Bug
> Components: java
> Affects Versions: 1.7.6
> Reporter: Jim Pivarski
> Fix For: 1.8.0
>
>
> According to http://avro.apache.org/docs/1.7.6/spec.html#Schema+Resolution ,
> writer and reader schemae should be considered compatible if they (1) have
> the same name and (2) the reader requests a subset of the writer's fields
> with compatible types. In the Java version, I find that the structure of the
> fields is checked but the name is _not_ checked. (It's too permissive; acts
> like a structural type check, rather than structural and nominal.)
> Here's a demonstration (in the Scala REPL to allow for experimentation;
> launch with "scala -cp avro-tools-1.7.6.jar" to get all the classes). The
> following writes a small, valid Avro data file:
> {code:java}
> import org.apache.avro.file.DataFileReader
> import org.apache.avro.file.DataFileWriter
> import org.apache.avro.generic.GenericData
> import org.apache.avro.generic.GenericDatumReader
> import org.apache.avro.generic.GenericDatumWriter
> import org.apache.avro.generic.GenericRecord
> import org.apache.avro.io.DatumReader
> import org.apache.avro.io.DatumWriter
> import org.apache.avro.Schema
> val parser = new Schema.Parser
> // The name is different but the fields are the same.
> val writerSchema = parser.parse("""{"type": "record", "name": "Writer",
> "fields": [{"name": "one", "type": "int"}, {"name": "two", "type":
> "string"}]}""")
> val readerSchema = parser.parse("""{"type": "record", "name": "Reader",
> "fields": [{"name": "one", "type": "int"}, {"name": "two", "type":
> "string"}]}""")
> def makeRecord(one: Int, two: String): GenericRecord = {
> val out = new GenericData.Record(writerSchema)
> out.put("one", one)
> out.put("two", two)
> out
> }
> val datumWriter = new GenericDatumWriter[GenericRecord](writerSchema)
> val dataFileWriter = new DataFileWriter[GenericRecord](datumWriter)
> dataFileWriter.create(writerSchema, new java.io.File("/tmp/test.avro"))
> dataFileWriter.append(makeRecord(1, "one"))
> dataFileWriter.append(makeRecord(2, "two"))
> dataFileWriter.append(makeRecord(3, "three"))
> dataFileWriter.close()
> {code}
> Looking at the output with "hexdump -C /tmp/test.avro", we see that the
> writer schema is embedded in the file, and the record's name is "Writer". To
> read it back:
> {code:java}
> val datumReader = new GenericDatumReader[GenericRecord](writerSchema,
> readerSchema)
> val dataFileReader = new DataFileReader[GenericRecord](new
> java.io.File("/tmp/test.avro"), datumReader)
> while (dataFileReader.hasNext) {
> val in = dataFileReader.next()
> println(in, in.getSchema)
> }
> {code}
> The problem is that the above is successful, even though I'm requesting a
> record with name "Reader".
> If I make structurally incompatible records, for instance by writing with
> "Writer.two" being an integer and "Reader.two" being a string, it fails to
> read with org.apache.avro.AvroTypeException (as it should). If I try the
> above test with an enum type or a fixed type, it _does_ require the writer
> and reader names to match: record is the only named type for which the name
> is ignored during schema resolution.
> We're supposed to use aliases to explicitly declare which structurally
> compatible writer-reader combinations to accept. Because of the above bug,
> differently named records are accepted regardless of their aliases, but enums
> and fixed types are not accepted, even if they have the right aliases. This
> may be a separate bug, or it may be related to the above.
> To make sure that I'm correctly understanding the specification, I tried
> exactly the same thing in the Python version:
> {code:python}
> import avro.schema
> from avro.datafile import DataFileReader, DataFileWriter
> from avro.io import DatumReader, DatumWriter
> writerSchema = avro.schema.parse('{"type": "record", "name": "Writer",
> "fields": [{"name": "one", "type": "int"}, {"name": "two", "type":
> "string"}]}')
> readerSchema = avro.schema.parse('{"type": "record", "name": "Reader",
> "fields": [{"name": "one", "type": "int"}, {"name": "two", "type":
> "string"}]}')
> writer = DataFileWriter(open("/tmp/test2.avro", "w"), DatumWriter(),
> writerSchema)
> writer.append({"one": 1, "two": "one"})
> writer.append({"one": 2, "two": "two"})
> writer.append({"one": 3, "two": "three"})
> writer.close()
> reader = DataFileReader(open("/tmp/test2.avro"), DatumReader(None,
> readerSchema))
> for datum in reader:
> print datum
> {code}
> The Python code fails in the first read with
> avro.io.SchemaResolutionException, as it is supposed to. (Interestingly,
> Python ignores the aliases as well, which I think it's not supposed to do.
> Since the Java and Python versions both have the same behavior with regard to
> aliases, I wonder if I'm understanding
> http://avro.apache.org/docs/1.7.6/spec.html#Aliases correctly.)
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