Can you give an example of maintaining the state of a batch apex?

Here’s a worked example: a batch that processes orders, tallies a running total of order amounts, collects every failed-record error, and emails a summary at the end. Everything that accumulates across chunks is an instance field that only works because of Database.Stateful.

The class

public class OrderTotalsBatch implements
    Database.Batchable<sObject>,
    Database.Stateful
{
    // State that survives across executes — needs Database.Stateful
    public Decimal totalRevenue = 0;
    public Integer totalProcessed = 0;
    public Integer totalErrors = 0;
    public List<String> errorMessages = new List<String>();
    public Datetime startTime;

    public Database.QueryLocator start(Database.BatchableContext ctx) {
        startTime = Datetime.now();
        return Database.getQueryLocator(
            'SELECT Id, Total__c, Status__c FROM Order__c WHERE Year__c = 2026'
        );
    }

    public void execute(Database.BatchableContext ctx, List<Order__c> scope) {
        for (Order__c o : scope) {
            try {
                // Accumulate revenue across chunks
                totalRevenue += (o.Total__c == null ? 0 : o.Total__c);

                // Update each order
                o.Status__c = 'Reconciled';
                totalProcessed++;
            } catch (Exception e) {
                totalErrors++;
                errorMessages.add('Order ' + o.Id + ': ' + e.getMessage());
            }
        }
        update scope;
    }

    public void finish(Database.BatchableContext ctx) {
        Long duration = (Datetime.now().getTime() - startTime.getTime()) / 1000;

        AsyncApexJob job = [
            SELECT CreatedBy.Email FROM AsyncApexJob WHERE Id = :ctx.getJobId()
        ];

        Messaging.SingleEmailMessage msg = new Messaging.SingleEmailMessage();
        msg.setToAddresses(new String[] { job.CreatedBy.Email });
        msg.setSubject('Order Totals Batch Complete');
        msg.setPlainTextBody(
            'Orders processed: ' + totalProcessed + '\n' +
            'Total revenue: $' + totalRevenue + '\n' +
            'Errors: ' + totalErrors + '\n' +
            'Duration: ' + duration + ' seconds\n\n' +
            (totalErrors > 0
                ? 'First 10 errors:\n' + String.join(firstN(errorMessages, 10), '\n')
                : 'No errors.')
        );
        Messaging.sendEmail(new Messaging.SingleEmailMessage[] { msg });
    }

    private List<String> firstN(List<String> list, Integer n) {
        if (list.size() <= n) return list;
        List<String> top = new List<String>();
        for (Integer i = 0; i < n; i++) top.add(list[i]);
        return top;
    }
}

What’s stateful — and what’s not

The same class without Database.Stateful

// Without Database.Stateful
public class OrderTotalsBatch implements Database.Batchable<sObject> {
    public Decimal totalRevenue = 0; // resets every execute!
    // ...
}

After running, finish would show totalRevenue = 0 (or the last chunk’s value only) and errorMessages = []. Every chunk got a fresh instance, every increment was thrown away.

Other common stateful examples

• Per-account rollups — Map<Id, Decimal> accumulating totals across chunks.
• High-water marks — oldestDate, largestAmount updated as more records arrive.
• Summary by category — Map<String, Integer> counting record types.
• Detailed per-record results — list of “succeeded”, “failed”, “skipped” rows for an end-of-job report.

Common interview follow-ups

• Are static variables stateful in batch? — No. Statics reset every transaction. Only instance fields plus Database.Stateful work.
• Performance impact? — Slight. Salesforce serializes the instance between chunks. Don’t add it if you don’t need it.
• Can I store anything? — Anything serializable — sObjects, custom Apex types, primitives, maps, lists. Avoid transient fields.

Verified against: Apex Developer Guide — Database.Stateful Interface. Last reviewed 2026-05-17.