Nebula Ingestion DDL

YAML is powerful way to express configurations, it's easy for people to understand and change. At same time, remember all different configurations and concepts can pose high tax when we starts support functions and preprocess, indexing, or consistent hashing (possible concept to expand/shrink storage cluster) This may lead to invent new set of configuration and concepts that only expert can remember.

Moreover, OLAP system is working as part of big data ecosystem, be able to transform and pre-process during ingestion time will provide an edge compare to other OLAP engines for user to adopt.

Use an inspiring example that not yet supported by nebula.

User has a hive table and a kafka stream ingest into nebula. Hive table has hourly partition keeping last 60 days of moving average of business spent per account; kafka stream contains business transactions in foreign currency of each account. User want to investigate account spending status in near realtime in home currency (e.g USD)

The complexity of this use case comes from three folds

• hive table may read data and eventually shard per account basis
• kafka stream may need to rpc and convert currency into usd
• both kafka stream may need to do stream/table join on per account basis before land result to run slice and dice.

If user write a RDBMS query, it should look like

OPTION 1 Materialized View with schema as part of config

create view nebula.transaction_analytic as (select accountid, avg(spend), transactionid, TO_USD(transaction_amount) from hive right join kafka on hive.account = kafka.acount where <all configs on hive, kafka>)

Alternatively, we can support two statement flow like

OPTION 2 Full Table with schema inference

DDL ` // mapping of hive table sync to nebula table create table hive.account ( accountid bigint PRIMARY KEY, spend double, dt varchat(20) UNIQUE NOT NULL ) with ();

create table kafka.transaction ( transactionid bigint PRIMARY KEY, accountid bigint not null, transaction_amount double, _time timestamp ) with ();

create table transaction_analytic ( accountid bigint PRIMARY KEY, avg_transaction double, transaction_amount_in_usd double, _time timestamp ) with (); `

DML insert into transaction_analytic select accountid, avg(spend), transactionid, TO_USD(transaction_amount) from hive right join transaction on hive.account = transaction.acount;

https://github.com/varchar-io/nebula/issues/180

The git history and code are messy. I ran tests for creating timeline query by day and week since you can do those with constant bucket sizes. An issue that arises is that the table below the timeline will show a negative value for the initial time because the start of a day/week may be before beginTime.

To implement month/year (which needs consideration of leap years / 29, 30, 31 days), likely need to write a UDF. I'm looking into it but still a bit confused on how to do this. Any formatting / code structure feedback is appreciated.

Currently Nebula defines file system interface in its storage component. Under this interface, there are a few implementations such as

1. local file system
2. S3 file system

There two are mostly used so far, now we're asking a new support to allow Nebula read and write data with GCS. This will be needed for nebula deployment in Google Cloud Platform.

(Context, this is how the current timeline works) The timeline query will have 3 basic inputs: start time, end time, and window size, simply it just slices the whole time range into pieces according to the window size.

For general timeline, this works great as long as the window size is flexible, but there are real business scenarios that they want the timeline to fit into the boundaries of a given unit, such as Weekly, Monthly, Quarterly, or Yearly.

In these cases, data needs to fit into the boundary of the time unit, and should not cross multiple different units.

The change should be simple in the query building/planning phase, but need to introduce a new interface and enough tests.

(Suggestion: A good starting task for new committer.)

Replace paths_ with a new object, such as FileSplit which should capture {file path, offset, length} and these should be part of the spec identifier. @ritvik-statsig

I0214 06:39:03.635987 18 Dsl.cpp:226] Query output schema: STRUCTflag:BOOLEAN,stamp.MAX:INT128 w/ agg columns: 1 I0214 06:39:03.636165 18 Dsl.cpp:354] Nodes to execute the query: 1 *** Aborted at 1613284743 (Unix time, try 'date -d @1613284743') *** *** Signal 11 (SIGSEGV) (0x0) received by PID 1 (pthread TID 0x7fe177d5d700) (linux TID 16) (code: 128), stack trace: *** (error retrieving stack trace)

Currently we support roll spec with time MACROs (date, hour, min, second) by specifying MACRO pattern in time spec. I think we should decouple this to get a better flexibility.

For example, below spec should be a legit spec:

test-table:
  ...
  source: s3://xxx/{date}/{hour}/
  time:
      type: column
      column: col2
      pattern: UNIXTIME_MS

This spec is basically asking us to scan s3 file path with supported macros in it, but the time is actually from an existing column. My understanding is we don't support MACRO parsing if we don't specify it in time spec.

cc @chenqin

While I'm updating this piece of logic - I did some refactoring to moving things around, mostly try to relocate logic into common as much as possible especially if it is common.

The change will support macro on source only and try to materialize it with supported macro (time related). Time spec will not bind to MACRO any more but can use its watermark as its time value (file level).

steps:

• clone latest code from master and build it locally
• run local stack by invoking ./run.sh
• go to http://localhost:8088

in the test set, run hist(value) only, the value distribution doesn't meet the expectation, I think value column should be even distributed as it's random generated in range [-128, 128] - https://github.com/varchar-io/nebula/blob/c8cdd6b9d6b74a76f1d6a70e7deb65a423eb4013/src/surface/MockSurface.cpp#L44

so I believe every bucket should have similar number of values in the histogram chart.

Also this query (value > 90) should produce non-zero buckets for value greater than 90, some bug out there! :)

I have tried this change on a fresh new ubuntu 18 twice, it works fine. Could you follow dev.md from beginning until step 'build nebula", hope this time it works alright.

I found the same view has incorrect height on my laptop while it is okay showing on a dell monitor.

Also - the main color for icicle/flame is too dark (dark red), not good for this visual, should do some adjustment on it.

UDTF support is needed, the first use case good to test is the Word Cloud query.

In the Word Cloud graph, users specify a "literal/sentence" column to rank all words in it, this query can be represented as

    select (explode(split(text)), sum(value))

The initial design will be:

1. Introduce the UDTF function and make sure the client can pass it in.
2. Note that, explode only can be applied to a LIST-typed field. In the above case, the "text" is not, but the "split(text)" is.
3. Our current interface that defines query keys needs to be updated to match (nebula.proto)

Implementation: ComputedRow is the interface to bridge internal data blocks to the computing buffer, internally it will invoke UDTF to gain a list value through the ValueEval interface. We should cache it for the first time, and introduce a has more interface to let BlockExecutor to loop over all of its combined values.

Every UDTF should maintain a cursor until the last combined value is served, hasMore shall return false.

Bonus: During the time of developing this new feature, we will also battle testing the support of LIST type which we haven't tested thoroughly yet. And clearly LIST/ARRAY could be a top-level supported column type after to embrace a lot of use cases.

Help Wanted This is super interesting work, but will also require some time to flesh out all the details. It would be great if any one is willing to contribute, and I'd love to work together collaborating on this, and offer as much help as I can.

currently, the histogram has little information for a client, a few enhancements we should have for more intelligent clients:

1. Integer column should report distinct values beyond current [min, max, sum, count] which is already good.
2. String column has a poor count of values only, for now, we definitely need distinct values for the client to decide if it should be used as a suitable dimension. If possible, getting average string length will be good info too.

In fact, having distinct values is great for the backend as well since it can automatically switch to the dictionary encoding to save massively on expensive memory consumption. So this will be very meaningful and useful work to be added to the Nebula engine, basically, we should auto switch to dictionary encoding when sealing each data block for all INT + STRING columns.

In the multipath scenario - currently, we skip any bad file that is included in a spec, we use continue to continue ingestion. But sometimes, users may want to fail the whole spec if any file is bad, rather than continue.

We need to introduce a flag to allow users to specify that behavior in table settings. @ritvik-statsig

Internally let's use UNIX MS (internal type size_t/int64) to store its value. But in the metadata layer, we could mark a column as Datetime so that we could apply different UDF on this type of columns, such as

• date - date of the month
• day - day of the week
• month - month value of the date-time
• quarter - quarter value of the date time

As an alternative, we could use JS function to implement these UDF, however, quickjs isn't accurate on the test results unless we import more advanced date lib into quickjs to support Date related function. Performance won't be as good as native implementation though.

Hey @ritvik-statsig,

Just try to repro with the file you send to me, unfortunately, I can not repro it. Pls, check the config I use and compare yours and see if there is any possible guess.

This is for investigating issue #163

Steps:

1. copy the file you sent to /tmp/oom.snappy.parquet
2. add below config to use latest code to launch single-node nebula cluster
3. watch:

Data loaded correctly without issue as seen in Node log:

I0302 22:41:05.525804 1830744064 TaskExecutor.cpp:128] processed task: macro.test@/tmp/[email protected]_I
I0302 22:41:05.527215 1830744064 IngestSpec.cpp:466] Ingesting from /tmp/nebula.lNqmqH
I0302 22:41:05.539216 1830744064 IngestSpec.cpp:547] Push a block: [raw: 817096, size: 1634020, allocation: 1634020, rows: 884, bess: 0]

So the sample file only cost 800KB of memory with 884.

Then I started web UI, the query is working alright -